Fishman Island has terribile villains and zero tension,but the flashback was maybe the best and the deepest of the entire series. But,compared to the hype,we can say FI didnt really live up to the hype. Punk Hazard gets trashed often,but honestly,especially when you re-read the beginning,its the only post-timeskip arc when you get a big sense of mystery! The SHs arrive there and you get this island with two opposite weathers,this scientits who you dont know who he is etc. so the beginning of Punk is well done in giving you hype! But we can say that in the end it didnt really live up to the hype,like FI. But at least both FI and PH are not very long as an arc. Dressrosa,instead,its more epic than PH and FI but its definitely too long. Wano has the same problem x10! But Wano is better than Dressrosa imho,so i vote for Dressrosa as worst arc.
Zou and Reverie are good but not really two arcs,and Whole Cake Island is good because its very SHs-centered and we dont lose time with some population to save,but the final part was too long as well. So i vote for Dressrosa,sorry Doflamingo,you are the best villain anyway!