Yeah, Nihilus wiped out a whole planet, thats pretty evil and brutal. Although so did Tarkin, but he didn't kill Leia when he had the chance, so I guess I will change my answer to Nihilus. If Tarkin had killed Leia, I probably would not have changed it. Kadesh, your right, Nihilus just doesn't care who or what you are, your as good as dead.
I still stick with my original assessment and go with Tarkin. The Grand Moff Tarkin who barked a command at DARTH VADER (!) - the same Vader who has no problem strangling every Imperial and their mother (regardless of their rank) - and ordered the destruction of an entire planet simply because he felt like it. He believed Leia's lie, he had no reason to continue with the operation - he just did it to be an evil bastard.
Grievous comes in second. Read the RotS novelization or LoE for details.
Malak is third.