Shaymin

The flowers on Shaymin's body burst into bloom if it is hugged with love.  It can dissolve toxins and transform a ruined land into a field of flowers in just seconds.
Shaymin is a cute hamster or hedgehog of some sort.  It has a second form, but you need to have an item called the Gracidea Flower to change Shaymin's form.  This item can only be obtained through a special event that's long gone right now, and there doesn't appear that another event like this is happening any time soon.  Anyway, when Shaymin does change forms, it becomes a Grass-and-Flying-Type.  Shaymin's Land form has all of its stats with a base of 100.  Base 100 for HP, Base 100 for Attack, Base 100 for Defense, and so on.  The Sky form has different stats.  This form's stats have a base ranging from 100-127; just including the HP, Attack, Sp. Attack, and Speed stats.  The Defense and Sp. Defense stats have bases of 75.  According to stats, the Land form is the stronger one.
Shaymin has a move exclusive to itself.  It's called Seed Flare, which has a base power of 120.  It may lower the target's Sp. Defense stat by about 3 stages!  Moves that only the Land form can learn are Synthesis, Aromatherapy, and Healing Wish.  The Sky form has moves exclusive to itself as well, which includes Quick Attack, Air Slash, and Leaf Storm.  Otherwise, excluding moves it can only learn from Generation IV, the two forms of Shaymin learn the same moves.  Shaymin has plenty of recovery moves, status condition healing moves, and STAB moves.  This makes Shaymin a somewhat offensive Pokemon.
This Pokemon's ability is Natural Cure.  It heals all status conditions just when you switch out Shaymin.  This includes stuff like Poison, Paralysis, and all of that stuff.  A great ability for keeping Shaymin in battle for a long time.
So the final verdict points to Shaymin being a good Pokemon, whether it's a Staller, or just a Pokemon you use to win battles, this is a Pokemon you should use if you can get your hands on it.

Giratina

It is said that Giratina lives in a world on the reverse side of ours, where common knowledge is distorted and strange.
Giratina is the mascot of Pokemon Platinum version.  He has uber written all over him.  His lowest stat is his Speed, with a base of 90.  He has a base HP stat of 150, an Attack with a base of 100, a base Defense stat of 120, a base Sp. Attack stat of 100, and a Sp. Defense stat of 120!  Giratina can also change his form when it is given the Griseous Orb.  This form has similar stats.  The HP and Speed stats still have their usual bases, HP having a base of 150 and Speed having a base of 90.  But the Attack and Sp. Attack stats have bases of 120, and the Defense and Sp. Defense stats have bases of 100.  It's just a small stat base number swap or something.
Being a legendary, Giratina has a large movepool.  Because it has two types, it has a wider variety of STAB moves, like Ominous Wind, Shadow Ball, Dragon Claw, Shadow Claw, Draco Meteor, and a move exclusive to only Giratina, Shadow Force.  All in all, Giratina has a good movepool as well as having good stats.
Giratina's ability is one common amongst many legendaries.  It's Pressure.  When an opposing Pokemon hits you with a move, the PP for it goes down by 2 instead of 1.  Not too good, not too bad.  It can backfire if your opponent uses a move like Trump Card, which gets more powerful the less PP it has.  I guess you could say it's a small price to pay for Giratina's awesomeness.
So the final verdict?  Giratina is a strong Pokemon.  It's a Dragon-and-Ghost-Type; it has a unique type matchup.  It's strength can't even be compared to anything.  Giratina is boss.

Basculin

Usually, the two different types of Basculin get along so poorly, that they'll start fighting almost instantly, as they are very hostile.  However, sometimes members of one school of Basculin mingle with the other members of another school.
Basculin is based off of your typical fish, though it seems like it could be something you would see labeled as "aggressive" at the pet store.  Based off of stats, Basculin is strictly an "in the middle" kind of Pokemon.  To start off, his two highest stats are his Attack and Speed stats.  However, he has no stats that have bases lurking in the triple digits area, but his Speed stat is pretty close to that.  Basculin's two lowest stats are its Defense and Sp. Defense stats.  It doesn't really let Basculin take hits, so it doesn't add up to Basculin being the number one Pokemon.  He has an okay HP stat, though.  I guess Basculin isn't too bad.
Basculin has some fair moves like Crunch, Chip Away, Flail, and Thrash- if you want to be daring.  This guy has a move called Final Gambit.  Basculin faints, but deals the same amount of damage as the amount of HP the user (Basculin), lost.  Basculin has some other okay moves, such as Agility, Revenge, Swift, Surf, Waterfall, Dive, Muddy Water, Hidden Power, Ice Beam, Scald,  Toxic, and Attract.
 Basculin's two abilities are Adaptability and Reckless.  Adaptability powers up your STAB moves.  It's a good power boost to Water-Type moves, as Water is already Basculin's STAB type.  Reckless powers up moves with recoil damage, but because the recoil moves do more damage, you receive more recoil damage.  Your only recoil move is Take Down, so I prefer Reckless.
Basculin is NOT the number one Pokemon at all.  However, it isn't too bad, thus letting Basculin have a little bit of potential.

Gastrodon

A long time ago, Gastrodon's back was shielded with a sturdy shell.  Traces of this shell are left in its cells.  It has no bones in its body.  If any parts of its body fall off, they will grow back again.
Gastrodon is a Pokemon that seems to be based off of a sea slug.  It's two forms make it somewhat unique.  It has fair stats.  Gastrodon's highest stat is its HP stat, with a base of 111.  He has in-between Attack, Defense, Sp. Attack, and Sp. Defense stats; they're okay in general.  The only flaw in Gastrodon's stats is its low Speed stat.  Overall, this Pokemon's stats aren't too bad, but could use some slight improvement.
Here's my favorite part of Gastrodon.  It can use Recover, which restores some of its HP.  A recovery move on a slow Pokemon like Gastrodon is pretty good, but be careful when using it.  Gastrodon can use Muddy Water, which is similar to Surf, but it can sometimes lower your opponent's accuracy.  Not too bad of a move, I know I would use it.  By breeding, Gastrodon learns some wicked moves, like Trump Card, Memento, and Fissure.  Gastrodon also learns Counter and Mirror Coat.  Counter sends back any Physical moves your opponent used on you, dealing twice the damage.  Mirror Coat is the same thing, but it just bounces back Special attacks.  Seeing as how Gastrodon is slow, this move is good to have on it, because if you move first, you'll have nothing to Counter or Mirror Coat against.
This Pokemon's abilities are Sticky Hold and Storm Drain.  If you have Sticky Hold, your item can't be taken.  This counts with moves like Knock Off and Switcheroo.  A good ability, especially when you have a good item on Gastrodon, such as the Quick Claw, which occasionally lets you move first.  Considering how slow Gastrodon is, it's an item you don't want to lose.  Storm Drain is like a Water-Type Lightningrod ability.  You take no damage from Water-Type moves.  You're part Ground-Type, so Water-Type moves aren't "Not Very Effective" on you.  A decent ability, but personally, I'll stick with Sticky Hold (I really bad at comedy).
So Gastrodon is a good Pokemon overall.  In general, this thing isn't the best, but it isn't the worst.  When I say worst, I'm talking to YOU Tynamo...

Rotom

Rotom is the Plasma Pokemon; its entire body is composed out of plasma.  Because it has a body like this, it can enter some types of machines, take control of them, and start making trouble.
Did you know that "Rotom" spelled backwards is "motor"?  This Pokemon is cute, and most of its alternate forms are also cute.  These forms include the Wash Rotom, the Heat Rotom, the Mow Rotom, the Frost Rotom, and the Fan Rotom.  The Wash Rotom is an Water-and-Electric-Type.  It looks like a washer.  There's Fan Rotom, which is an Electric-and-Flying-Type.  Obviously, this one looks like a fan.  Next is the Mow Rotom, which is an Electric-and-Grass-Type.  This one looks like a lawnmower.  The Frost Rotom, which is part Electric and part Ice, mimics the appearance of a freezer or a refrigerator.  Last off is the Heat form.  This one is an Electric-and-Fire-Type and looks like an oven of some kind.  The Normal Rotom is just an Electric-and-Ghost type.  As far as stats go, the Normal Rotom has acceptable Defense, Sp. Attack, Sp. Defense, and Speed stats, but has low HP and Attack stats.  The alternate forms of Rotom have a low HP stat.  Since they've changed form, their Speed stat is lowered a little bit, but isn't too bad in general.  They have boosted Attack, Defense, Sp. Attack, and Sp. Defense stats, but it still has an arguably low Attack stat.
All the Rotom forms, except the standard one, have their own exclusive moves that only they can learn.  The Heat Rotom has Overheat, the Mow Rotom has Leaf Storn, the Frost Rotom has Blizzard, the Fan Rotom has Air Slash, and the Wash Rotom has Hydro Pump.  These alternate forms are different, so they have different STAB moves.  Rotom has a pretty decent movepool, giving it some pretty robust moves, so the movepool is good.
Your ability is Levitate, which makes you completely immune to Ground-Type moves.  All of the Rotom forms are part Electric-Types, so they're weak to Ground-Type moves.  This ability is great on Rotom.  Well, not on Fan Rotom.  It's a Flying-Type, so it's already immune to Ground-Type moves.  Oh well, I guess that a Pokemon as useful as Rotom has at least ONE flaw.
So Rotom is a good Pokemon, especially when you change its form.  This a Pokemon you probably don't want to pass up.

Deoxys

An alien virus that fell on earth on a meteorite went through a DNA mutation which made up this Pokemon.  When Deoxys changes its form, an aurora appears.  The crystal formation on its chest is its brain.  This Pokemon is highly intelligent and wields psychokinetic powers.
Deoxys is the best thing to happen to Hoenn.  Basically, this thing is probably one of the strongest and most capable of all the Pokemon of Hoenn.  It can change into 3 powerful alternate forms.  The first form is Deoxys's Attack form.  This makes Deoxys have an extremely boosted Attack and Sp. Attack stat, with a base of 180!  Like all the other Deoxys forms, Attack form has a low HP stat, and this form has really low Defense and Sp. Defense stats, with a pathetic base of 20.  The Speed stat has a base of 150, so you're certain to go first in battle.  The next form, which is the Defense form, has incredible Defense and Sp. Defense stats with a base of 160.  Of course, now, Deoxy's Attack and Sp. Attack drop all the way down to having only a base of 70.  I guess it isn't so bad when you think about it.  Again, this form has a low HP stat.  It's Speed is cut down to a base of 90, but again, this isn't too bad.  With highly boosted Defensive stats, you have to make sure that Deoxys isn't practically invincible; thus making the game too easy with Deoxys.  The Speed form has a high Speed stat, as if that wasn't completely obvious.  The Speed stat has a base of 180!  It has balanced out Attack, Defense, Sp. Attack, and Sp. Defense stats, and of course, a low HP stat.  Deoxys's Standard form has low HP, Defense, and Sp. Defense stats, and high Speed, Attack, and Sp. Attack stats.
Deoxys has its own move, exclusive to only itself.  It's called Psycho Boost.  It's a strong move, but your Sp. Attack stat is lowered by quite a few stages.  The Attack form has Superpower, a move that only it can learn.    The Defense form has moves exclusive to itself, like Counter, Mirror Coat, and Iron Defense.  Deoxys has an HP restoring move called Recover.  Only the Standard and Defense forms learn this moves.  The Speed form can learn Extremespeed, a move that always goes first.  In general, you can tell where this is going.  Deoxys can use tons of different moves.  It's an amazing Pokemon.
Deoxy's ability is Pressure.  If an opponent lands a hit on you, the move they just used has its PP decreased by 2 instead of 1.  Not my favorite ability.  Sometimes, this ability is useful, but it can also be very harsh if your opponent uses something like Trump Card, which gets stronger the lower amount of PP it has.  Its full PP or only of 5, so be careful.
Deoxys is one of the best Pokemon out there.  Capable of more things than almost any other Pokemon, Deoxys is going to say useful for a long time.

Castform

Castform changes shape varying on the weather.  It does this by using its uncanny ability of changing its cells.  It has been discovered that Castform's molecules resemble water.
What a Pokemon- something with the ability Forecast, which allows it to change not only its shape, but its type as well.  In the Sunny weather condition, Castform becomes a Fire-Type, and takes on a form that makes it look like a sun of some sort.  In Rainy conditions, Castform becomes a Water-type.  It looks like a rain drop or something.  Castform's final alternate form is a cold cloud of snow of some kind.  This transforms Castform into an Ice-Type.
Castform's base stats are all of 70.  All of its stats- HP, Attack, Defense, Sp. Attack, Sp. Defense, and speed- all have a base of 70.  This isn't completely good, but when you have a Pokemon that changes forms with a simple move like Rain Dance or Sunny Day, it'll just have to do.
You can keep Castform either Physically based or Specially based because of its balanced out stats.  One good way to use Castform is to pick one alternate form you like and stick with that.  If you pick the Sunny form, you'll have to give it Sunny Day to switch forms.  Give it some STAB moves like Fire Blast and Flamethrower, and try to include Weather Ball, along with another move of your choice.  If you want to use a Rainy form Castform, give it Rain Dance to make transform our little friend Castform.  After that, you could give it STAB moves like Scald or Hydro Pump, as well as including the move Weather Ball.  If you pick the Snowy Castform, use Hail to make it shape shift its appearance.  Give it STAB moves like Ice Beam or Blizzard.  Blizzard has its accuracy boosted 30% higher in the Hail weather condition.  That makes Blizzard have a 100% accuracy.  Of course, you'll probably want to give Castform Weather Ball.  Otherwise, you could make Castform have any other moves, or combinations of all 3 forms, or something else.  Other good moves Castform has are Future Sight, Ominous Wind, Hidden Power, Energy Ball, Thunderbolt, and other moves.
So besides the fact that Castform's stats are a bit cloudy (I'm bad at jokes), I'd say it's a pretty decent and cute Pokemon.

Unown

Unown's forms seem to have some meaning; resembling that of ancient hieroglyphs.  They are always stuck on walls.  When one Unown is all by itself, nothing seems to happen, but when there's more than one Unown, it said that an odd power of some sort emerges.

Unown has more forms than any other Pokemon with a total of 28 different variations.  One for each letter in the alphabet, and an exclamation point and a question mark.  Unown has a poor base stat of 48 for HP, Defense, Sp. Defense, and Speed.  It makes me wish Unown really was "unknown", because this is a really pointless Pokemon to have in the game.  Its Attack and Sp. Attack stats have a base of 72.  It seems like Game Freak doesn't want use Unown at all.  It also makes one wonder if this Pokemon was just made for the anime.  Again, Unown's stats are horrible.

Only one move.  Unown has only one move.  It's Hidden Power.  Its type and power is based off of the Pokemon using it.  To find out Unown's Hidden Power type, go to the Pokemon center in Mistralton City and ask a guy by the counter.  He'll tell you what type Unown's, or any other Pokemon's Hidden Power type.  Use Google to find more info on Hidden Power, because there's too much for me to explain in one post.  Having only Hidden Power makes Unown sink like a rock.  

Unown's ability is Levitate, which means you're immune to Ground-Type moves.  I guess it's okay, because you're immune to Fissure and Earthquake, but Unown isn't weak to Ground-Type moves, so the ability isn't really that useful either, and don't leave out the fact that nobody hardly even uses Unown anyway.  

Of all the Psychic-Types, Unown has to be the worst.  It's really cool that it has all those forms, though, not to mention the fact it's so cute!  But in battle, Unown is worthless.