Third evolution: Hydreigon.
It's a three-headed, well, hydra. That eats stuff.
Lots of stuff. It's also got cool typing and a great movepool and some pretty awesome stats. Too bad it's garbage in the competitive scene now... Oh yeah, its shiny form is pretty great too. I like Hydreigon.
Second evolution: Weavile.
It's actually my favorite Pokemon, but since its evolutionary chain only has two in it, I'll just put it here. It's got a cool appearance and I'm a sucker for dark-types. Not to mention it's also got some nice usability as a revenge-killer.
First evolution/unevolved: Klefki.
The thing is ADORABLE. Plus Prankster is a neat ability and caters to my "annoying as hell entry-hazards and screens" competitive playstyle. Insta-paralysis is good too. It's just overall a cool little thing.
also I can make @Supeh Mario really mad using it
Legendary: Darkrai/Giratina.
Darkrai is plain-out cool, what with Dark Void and all (I hate and love that move) and his appearance. Again, I like dark-types. Giratina is a ghost, which I also like as a type. Giratina's also cool because of the dual forms which take on very different roles in competitive, being either a wall or an offensive attacker. It's also got an amazing personal little world.