As characters go I think Akainu.
Akainu was great in Marineford, felt ruthless, cruel and the way he dealt with Ace and Luffy was great, still not a brainless warmachine but kept that bit of cowardice you see in some villains: he was ready to slaughter every pirate who step in Marineford still plotted to weaken WB as much as possible, fought with him until he was rendered unconscious or decided it was better to leave the fight and switched from Terminator mode to furniture in a moment when Shanks appeared. Great character performance, Oda did a good work. Post time skip he is reduced to Deskainu who can only shout from behind a desk and that character is actually worse than Akainu.
Kaido in the pre time skip wasn't even thought out properly, see his silouhette in Enies lobby, at first when introduced in the post time skip he seemed just a mindless brute and in Wano, an arc long a life age of man, he get almost nothing, no flashback, no real characterization, he just wants to fight for the sake of it, we know that despite that he had a sense of honor but not so adamant, he killed the old hag and the cp0 guy for ruining his duels still he didn't redo them from scratch, also he could have done like Kata and inflict himself an injury to level the field. But nonetheless he gets my simpathy for having to fight an immortal cockroach like Luffy who comes back in moments stronger and stronger defying all logic.
