Toriyama stated in the Daizenshuu books that Mystic/Ultimate Gohan is the strongest character unaided by fusion of all time. The only 3 characters stronger than him require fusion of some kind and that is Super Buu (Gotenks), Super Buu (Gohan) and Vegito. Super Buu after absorbing Gohan is Buu's strongest form. The confusion arises because of the anime. In the manga, Goku stated that he had no chance against Super Buu after he absorbed Gotenks but he said that at full power he could defeat Kid Buu. As well as this, he stated after Super Buu had absorbed Ultimate/Mystic Gohan, that even if they teamed up that they would be no match for him, so they desperately needed to fuse in order to become Vegito, in which Goku stated it was they only way for them to win. After absorbing Gohan, Super Buu became even stronger than before, making Super Buu much stronger than Kid Buu. Kid Buu may be more ruthless and dangerous than Super Buu but in terms of power he doesn't compare.