Definitely Buu. Even Dabra, Lord of the Demon Plane, was stronger than Cell (Goku explained this to Kibito Kai after he was surprised at Vegeta's typical arrogant comment about Dabra not being as hard as they thought), and Dabra's power level is nothing in comparison to Buu's. I'm afraid our green-skinned friend won't have very much of a chance against everyone's favorite pink blob.
buu would easily win.
kid buu
Who do you think would win?! CHUCK NORRIS!!
kid Buu
super buu
Fat Buu loses, Super Buu stalemates and Kid Buu curbstomps.
Wrong. Ultimate Buu would win. Gogeta has a weakness like Gotenks & he's weaker than Vegeto. SSj3 Gotenks is little close to Gogeta's power. Plus Gogeta is in the movies , not the show.
Cell.
Perfect Cell
Kid buu aparrently because if you watched dbz you would know because it is the Purest (therefore strongest) of all the Buu`s forms.
It is difficult to determine who would win between Darkseid and Kid Buu as they are both powerful villains from different universes (DC and Dragon Ball Z). Both characters possess immense strength, abilities, and powers that make them formidable opponents. Ultimately, the outcome of a battle between them would depend on various factors such as the circumstances of the fight and the writers' interpretation.
Super Buu would easily win whence fighting Goten in any of Goten's forms (Base, SSJ1, etc).