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.