Definitely Gohan. Pretty sure Gohan was stated by Toriyama to be the strongest unfused character, but even that aside, Gohan was pummeling Super Buu even with Gotenks absorbed, while Goku stated that he wouldn't be able to beat Super Buu. Not to mention that, unless Goku is dead, Super Saiyan 3 has a very limited time frame, Gohan has no such cause for concern.
ssj3 goku fought on par with kid buu which is his ultimate form
Kid Buu wasn't the ultimate form, iirc Kid Buu was actually one of the weakest by virtue of the fact that he had nobody absorbed and thus no extra power besides his own. He was rather the most dangerous because he was a raging berserker that would just destroy anything, unlike the childish Fat Buu or the more intelligent Super Buu.
I thought Gohan was going to be a little more beasty in his 'Mystic' form, don't get me wron though, he did some heavy damage. But in comparison, super sayain three Goku is a monster.
Gohan's feats were more impressive than SSJ3 Goku's, if Gohan ever looked weaker than Goku it's because the story had to allow for it in order to progress. Goku would not have done as much to Super Buu w/ Gotenks as Gohan did, and even if he could he would be very pressed for time since he can't keep Super Saiyan 3 up for long.
