I really like Diavolo, tbh. I feel he's a better version of what DIO was in part 3. We already know DIO's backstory and charisma, leaving a lot of the mystery aspect that part tried to build-up much weaker as a whole, only leaving the mystery of The World left for him. I also didn't really find his presence super intimidating throughout the part with how incompetent most of his assassins were building up to him
Diavolo, on the other hand, is the most mysterious main villain by far and the story effectively incorporates that into his plot and goals. He isn't mysterious for the heck of it, his whole main goal is to get rid of his past. It helps that every stand building up to him is effectively scary in some way, making it easier to think "wow, the boss must be super strong if even THESE GUYS can't go against him". And then the light twist in showing off King Crimson around 2/3rds in is a great turning point and, again, effectively builds up Diavolo as a tangible threat.
I also like how he represents the part 5 cast fighting for better fates, how his ability is really cool, how he has multiple personalities to make him all the more interesting and his design in general.
I know I spent the post gushing about him, but I'm not quite sure if I think I like him more than Funny Valentine, or possibly even Kira. Still, the main point is that I find him super underrated as a main antagonist.
