Super-Gorilla Grodd.... Plus, and I can't emphasize this enough, he's a talking gorilla.(There's also a spinoff thread where various people try to work out the parameters for what makes a Great Villain, but for my money the first thread is much more fun.)
The Shark, for using an "invisible yellow force field" for protection against Green Lantern. Because Green Lantern's ring can't penetrate anything yellow, even if it's invisible. That's just plain good comics.
[On Thanos and his motivations:] That kind of psychosexual shenanigan is just plain good comics.
Kang the Conqueror is a yutz, who has yutz written all over him, and whose DNA coils up into chromosomes that remarkably resemble the word "yutz". ...As for Yutz the Conqueror, the fact that he also wants to show up on the list in four different kinds of drag contributes to his yutzian nature.
[On why The Anti-Monitor shouldn't make the list:] he's only great by fiat, which is to say about as great as Doomsday or Bane (albeit much better drawn), and he's got one of the worst names in comics. (Shouldn't the Anti-Monitor just... I don't know, not watch people?)
the Red Skull is just coasting on that whole Nazi thing. It's like he doesn't even try any more.
[On why Shazam villain Mr. Mind is cool, despite literally being a tiny little worm:] Besides, you have to give it up for a worm so evil, the state electrocutes him.