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.