Seen a lot of love about for my favourites already - Axolotl FTW! - but I'm gonna have to throw out the Honey Badger.
http://en.wikipedia.org/wiki/Honey_badger
Saw a documentary about them, totally awesome creatures. Point one, they mate 'energetically' for 3 days. Point two, they've evolved the most fantastic defence mechanism ever. Not poisonous, not fast, not clever, not spiny, not camouflaged...they're just so tenacious and vicious, they're functionally unkillable. A hungry lion will walk past a honey badger, because it's not even worth trying to kill the damn things.
Bloody-mindedness as a defence! What's not to love about that?!?