1) The most popular villain
2) The most evil villain
3) The most powerful villain
4) The best written (rounded) villain
5) The villain with the best character design
6) Largest number of great villains
1) Clearly DC with Joker.
2) Really a hard question, cause it depends on how you find evil. You could probably make an argument for characters like Carnage or Doomsday who just kind of kill anyone for no reason, or characters like Thanos who are more the evil dictator type.
3) It depends on how you count villains who use various things to power themselves up. Like... you could make an argument for Dr. Doom or Thanos at certain points. You might be able to argue for Dr. Manhattan although his powers are somewhat vague but *spoilers* after Rebirth it seems he may be able to create multiverses which is pretty strong. Ultimately, I'll probably go with molecule man. Pre-retcon and post secret wars (spoiler) he's essentially omnipotent.
After that, probably either Galactus or pre-retcon beyonder. Beyonder was essentially god. Silver Surfer, who is more powerful than Superman, was created with a fraction of Galactus' power.
4) Dr. Doom. I feel Joker will be the most popular answer, and he has that sort of mysterious thing going, but I like villains who deep down feel like they are doing the right thing. A case can also be made for Magneto in this regard, but his writing has been way less consistent.
5) If you're talking about pure visuals, I really love the classic Venom design.
6) I read a lot more Marvel so I'm kind of biased, but Marvel.
