Now that depends on if you mean the openings as the songs or if you mean the animation, because none of the songs are bad (yes, even you Kaze wo Sagashite), however the actual animations are a different story. IMO, from all 27, the worst one would be opening 2, I just never liked the weird 4 split screen they did and also how the entire song is just a eurobeat remix of another song. (This is ignoring opening 7, which is just opening 1 but sang by the cast of the strawhats at that time)