Theoretically, Kars has 400 IQ, but he acted very irrational.
Dio? Well... he has one of the most powerful power in JoJo - time stop. If he wasn't cocky, he could easily kill Joestar group at the moment when they entered to his mansion.
Kira? Hmmm.... I think he is most consistent villain. He very well used Killer Queen powers (fight with Josuke). Of course, he suffer on typical main villain sickness - he is extremaly cocky, and due this, he said everything about himself to Hayato.
I don't know Diavolo and others, so I don't judge them.
Probably Funny Valentine, he literally commits no mistakes.
But all the villains are brilliant in their own ways though, if not Valentine it'd be a toss up between Dio and Diavolo. They were consistently outsmarting the jojos but had a fair deal of luck though. Their cockiness ultimately didn't affect their battles much.
Only Kira really entered the realm of committing a dumbass mistake that actually cost him victory.
Kars, and not just because of his IQ of 400. Remember his fight against Lisa Lisa and Joseph? Yeah, that speaks for itself, really. If not for Stroheim's intervention and icredible, dumb luck on Josephs part, Kars would have obtained the ultimate victory.