It had one of the best fights (action-wise) and one of the best and coolest characters (katakuri), but best arc?? HELL NO!!!!
Reasons:
Pacing was atrocious- up there with Dressrosa, can't decide which one is worse.
The shitty cake plot was basically the entire arc, taking up all most the whole arc- and most disappointingly the payoff was crap, only dropped Big Mom for a brief moment.
Hated the Vinsmokes- didn't like them as characters: they are wannabe evil antagonists and unlikable characters. Reiju might be the only nice and tolerable one- but even then i find her bland.
Year of Sanji?? Pfff-yeah right. WCI really didn't do much for sanji imo, the only real benefits he got was a raid suit. The extra backstories, his conflicts with Luffy-they are not really memorable imo. Personally i didn't feel like sanji needed a backstory, and his confrontation with Luffy is also weaker than Ussop's imo.
A lot of fake hype- Bobbins, Pound's death, the wedding cake etc.
Yes WCI had a lot of good stuff in it, but unfortunately the good don't outweigh the bad.
Katakuri was carrying the arc hard- without him the arc would be a lot worse imo.