Going to come out of left field for this, but in terms of who I think has a cool design or concept:

That's right, Casey Jones. As a fan of street level characters, I dig the fact that he's very much a product of a rough city. Armed with just a bunch of ordinary and common tools like baseball bats and hockey sticks, he faces wave after wave of gang members, Foot ninjas, and mutants. While he doesn't have a lot of training, he has enough. Combine that with a lot of guts and fortitude, and Casey becomes the common everyman underdog you'll root for no matter what.

Still, the one common denominator I like across all iterations of Casey is the mask. It's creepy, emotionless, tough, intimidating, and reassuring all at the same time.

Awesome design and character!