Crobat

After evolving from Golbat, Crobat got one more pair of wings- where its legs used to be.  Because of this, Crobat isn't exactly the best at walking, but he's a great when it comes to flying.  It's a Pokemon that becomes more active towards night, and flies very silently.
Crobat is the original fully-evolved Bat Pokemon.  He basically poops on Swoobat.  In other words, he's like the "dominant Bat Pokemon".  He has great stats.  Fine HP, Attack, Defense, Sp. Attack, Sp. Defense, it's pretty decent.  But the thing Crobat is probably best known for is its Speed.  A base Speed stat of 130 makes Crobat swift and evasive.  He's usually going to go first if he's against non-legendaries and stuff like that.  Again, thanks to balanced out Sp. Attack and Attack, he'll be fine with Physical or Special moves.
Crobat can use Cross Poison, Air Slash, Steel Wing and Zen Headbutt; many moves are good on Crobat.  He had really good tutor moves in 4th Gen.  Ominous Wind, Sky Attack, and Heat Wave are probably the 3 best.  The point is that Crobat has pretty good moves- not the biggest movepool, but an acceptable one.
Crobat has okay abilities.  One is Inner Focus, but I'm pretty sure people have better things to do than use Fake Out against you.  But you are protected from flinching from moves like Air Slash, so it isn't all bad.  Crobat's new ability only accessible through the Dream World is Infiltrator.  It ignores the effects of moves like Light Screen and Barrier have no effect.  It's not bad or anything, but I prefer Inner Focus.
Other than the frail abilities, there's nothing really bad to say about Crobat.  He's just- awesome...