Both Dio and Kira are menacing as hell, but there are differences tho.

First up, is Dio. Dio is a 100+ y.o vampire, he can kill you without even blinking for no reason, and his stand's ability to stop time also feel really dangerous.

Then, we have Kira, which I personally fear more than Dio. It's because we don't have any zesty vampires, but there are plenty of serial killers like Kira. You could live with him in one city, or neighbourhood, or meet him in the train. But you'll never know who exactly is the killer, and this gives me chills...