Kirby wins. He's fast enough to more than keep pace (being able to fight against Air Machines & Meta Knight's MFTL travel speed/Bullrushes), can take all of Buu's punishment thanks to his multiple showings against black holes. Add on Inhalation to deal with Ki blasts, all of his extra hax to work around Buu's HF, & physical punches being right on his level, and he should certianly win this imo.
Buu one shots
Kirby eats dat boi
Kirby wins. He's fast enough to more than keep pace (being able to fight against Air Machines & Meta Knight's MFTL travel speed/Bullrushes), can take all of Buu's punishment thanks to his multiple showings against black holes. Add on Inhalation to deal with Ki blasts, all of his extra hax to work around Buu's HF, & physical punches being right on his level, and he should certianly win this imo.