He lost to super buu while Goku owned kid buu in base form
OT: Gotenks wins power wise.. But because of Gotenks extreme cockiness and lack of experience, Goku takes it.. EXTREME-DIFF
Super Buu (Gohan Absorbed) is the MOST POWEFUL form of Majin Buu... Note, Goku alone CANNOT beat Super Buu..
He's able to kill Kid Buu alone, only thing that made Kid Buu SEEM more powerful was that he was dangerously unpredictable as Super Buu had intelligence, and actually cared about the prospect of battle before destroying his opponent, Kid Buu originally goes for PURE destruction.
Edit: Nah, Gotenks take it.. It's DBZ xD Skill doesn't matter all too much, the power gap is all that matters.