Character/backstory vise, Kaido for sure.
Generally speaking, most of the side characters in Wano were complete disappointment. Hawkins, Big Mom, Yakuza bosses, Oniwabanshu, samurai etc were all trash. Hawkins especially, dude has one of the most interesting abilities in the series, and pre TS had one of the highest bounties (3rd or 4th highest amongst supernova IIRC).........only to get offscreened by injured Law, and then low diff'd by Killer (aside from the hostage situation it was low diff).
Even most of the scabbards were wasted, after hyping them up so much. Yeah, they did some stuff on Kaido. But Oda could've had them deal with the Tobiroppo? Instead of wasting Page One and Ulti on BM for no reason, have Franky/Robin be unexplainably powerful enough to take on Sasaki/Black-Maria. What did Kyoshiro even really do to stand out, after being the most hyped Scabbard with an actual decent backstory in the arc?