I disagree on that.
While I dislike Kars, DIO in Part 3 seemed like he'd be an immense improvement. Always in the shadows, far less openly evil than Dio Brando. It seemed like he had been improved and received some natural development for a guy locked in a casket for 100 years. But, come DIO's World, he's back to just being evil and acting like a generic bad guy.
Yoshikage Kira is a guy who runs entirely on charisma, and it works. The only issue is that he exists only so Part 4 can have a villain that actually sort of works, small town serial killer. Every other villain was essentially just evil because they felt like it, Kira was the bad guy because it was in his nature to do so. He's like the Jeffery Dahmer of JoJo's villains, he's just doing what he can to get off.
Diavolo is a return to form for Araki. He is a villain fully entrenched in the themes of a part, Part 5 being about resolve and acting against fate. While Giorno and the crew work and struggle to their endpoint, showing immense resolve, Diavolo hides and skips time in an attempt to warp fate to his desired endpoint (which is revealed to be a pointless endeavor in Sleeping Slaves). Pic related explains why Diavolo's skipping is an important factor in the themes of the part, as it's an explicit way of showing his true lack of resolve.
Finally, there is Pucci, who is a really good antagonist. He once again embodies the themes of the Part and the series as a whole, his goal being to show the world their fate to achieve heaven. Every scene he's in he takes center stage, he has so much charisma and this is in addition to the Part showing more of the philosophical DIO we were promised back in Part 3 but never got.
I'm basically out of text now, but you get the point. Part 2 and 3 are where Araki dropped the ball, Part 4 is where he starts to pick it back up.