That is a hard one.

Cammy, being the agile and fit woman she is, must have a shapely but still fairly rounded butt based on her sprites. I’m not counting her model in SFIV because if I was then compare it to R. Mika’s sprites then there’d be no contest.

On the other hand, judging from her sprites and art, R. Mika, was meant to be a thicker woman (but still fit), which makes sense because her fighting style resembles that off of a heavy weight,  so I’d imagine her butt must be bigger in size and more plump.

As much as I like Cammy, all evidence points to Mika having the better ass butt in the end isn’t it all subjective? 

R. Mika also uses her butt as lethal weapons (plural because there’s two cheeks and I imagine each being just as dangerous) so there’s that too.