Bo ^ 7 and Don Patch (Yes, yes, I know that's the point).
Gamma fucking Akutabi. The dude can stop bullets, missles, and even has a demon mode that makes him invincible. Still badass, though.
Sosuke Aizen. As long as you see his release, he can do whatever he wants. His Zanpakuto is insanely broken. My guess is that they'll have to convince Tousen to betray him in order to defeat him.
Arale Norimaki (Yes, it's a gag manga, so it's allowed)
Kenshiro (Fist of the North Star) Enough said.
Washuu Hakubi: She can delete memories, created the most dangerous lifeform on the planet, and can do a perfect German Suplex (read the Manga). Plus a lot of other things.
Alucard: The dude has over a million people stored inside him. He's been decapitated, sliced to ribbons, Blown up, etc., and still can't be beaten.
Son Goku in Youkai form (Gensomaden Saiyuki). He caught a fucking bullet fired at point blank with his teeth. HIS TEETH.