Buu is easily the better of the two arc, the King Piccolo arc is my least favourite arc from Dragon Ball & Z (except maybe Search for the Dragon Balls but that's barely an arc). Might've just been because it was the last arc I ever experienced but the King Piccolo arc is the awkward teenage phase of the Dragon Ball series. It's not a good Dragon Ball arc because it lacks the comedy that makes those arc so enjoyable. And it's not a good Dragon Ball Z arc because it lacks the wider cast that makes those arcs so enjoyable. Like, in Dragon Ball Z arcs, the secondary cast do stuff; Goku only beat Raditz because of Piccolo & Gohan; Gohan, Krillin, & Yajirobe were all integral to defeating Vegeta; Vegeta killed most of the baddies on Namek; Vegeta killed 19, & Gohan killed Cell; Goku only beat Buu because of Vegeta, Raditz, & Buu. In King Piccolo, Tien just got his ass-kicked and everyone else died. Yajirobe did beat one baddie I believe, but it's just the Goku show in King Piccolo. Dragon Ball needs the humour to make that work.
I mean, Goku vs. King Piccolo is still a pretty good fight with an iconic finish so it's not awful but definitely the weakest. Buu's not perfect though, the middle of that arc is a pretty boring slog with some weak fights tbh, but the bread of that arc is pretty scrumptious. Love Videl, love Vegeta & Hercules character arc, love Candy Vegito, love the ending.