Probably Viral from Gurren Lagann. His theme is great and it fits him perfectly.
I have a bunch of others that I like a lot though, so I'll just list and link them.
Kira's theme from Death Note, which was essentially the "shit's going down" music for Kira stuff.
Zero's theme from Code Geass, which played whenever Lelouch/Zero was pulling off some of his crazy shenanigans.
Raygou's theme from Kill la Kill
Kefka's final boss music from Final Fantasy III/VI (stupid international numbering confusion) is a mix of all of his various theme songs throughout the game and is pretty awesome.
Ridley's theme from Super Metroid is neat, I've got tons of nostalgia attached to it.
Frog's theme from Chrono Trigger is great and another one that is nostalgia-laden for me, and all the other character themes from Chrono Trigger are also great, so that's a plus.
Although it's a boss theme and only shows up the one time you face it, the
Taurus Demon theme from Dark Souls is great and gets me hyped the fuck up whenever I hear it.
The Rains of Castamere is kind of the Lannister family theme in Game of Thrones, rather than being for a specific character, but it's good so I'm listing it anyway.
And I think that's enough of me being an asshole and linking every character theme I could think of that I liked.