I personally think it's Kittan.

I personally like Viral. It's hard to write a character like him, especially when his motivation is just 'do my job, get my hat back'.

Personally, I kind of like Rossiu the best. While he's only a sub-fighter in part one and a powerless dick for a good bit of part two, you gotta admit he got shit done (even though the whole execution thing took it to far). Admittedly, part of the reason I like him was that his VA was the same as Lelouch for Code Geass and it really shows at times.

I'm a main person guy so it's Simon. But Rossiu is cool being voiced by the actor of both Lelouch and Ichigo.

Mmmmm... Kittan, hands down.

Sure, Simon and Kamina are the stars of the show, and without them humanity wouldn't have gotten anywhere; the same can even be said for Rossiu. But Kittan was always there in the background, being awesome and laying the groundwork so that the rest of Team Gurren/Dai-Gurren could succeed.

First guy to steal a Gunman without aid from another mech? The guy who rounded up the rest of the supporting cast not from Jiha or Litner? The one who filled Kamina's shoes while Simon was stuck in his funk? One of the few people shown to be competent at their jobs in the new world order? The one who reconvened a dispersed Team Dai-Gurren? The third known character to master Spiral Power, pull off a Giga Drill Breaker, and prove, beyond a shadow of a doubt, that theoretical calculations and odds are meaningless to humanity? Yeah, Kittan's got it all in Spades.