Mr. Freeze, Harley Quinn and Mystique. I’ve never read a story, in which they were villanous, that wasn’t at the least entertaining.
The Rogues were my favorite villains (DCU, mostly under Wally’s run), Captain Cold especially. Not the nicest guy in the world, but when written well, he’s a great leader who cares about the people around him and has a strong personal code.
Jason Wyngarde (aka Mastermind). Woefully underutilized, powerful beyond measure, former right hand of Magneto, snazzy dresser, sex symbol for the ages. He’d ugly and twisted and awesome, and nobody loves him but me.
Captain Cold and the Shocker, for similar reasons. The idea of blue collar villains who treat it like an actual job interests me, when so many enemies just shift their motivation to “Curse you XXXXX, I shall have my revenge!” sooner or later.
SOLOMON GRUNDY. Basically a cross between the Hulk & a Zombie, he is a tragic, but super powerful creature. A great character for we Monster lovers—perfectly suitable in a slugfest or as a tragic figure (like Frankenstein’s Monster).
Leady Deathstrike was always one of my faves. I used to love those Silvestri Wolverine issues. ;)
I would have to go with Deathstroke. He’s fought just about everyone in the DC Universe and even took down the Justice League in less than 10 seconds in the pages of Identity Crisis. Plus he’s way more badass than Deadpool, who was a blatant rip off.
Luthor. I love a villain that can stalemate a hero, and I like the idea that with all Superman’s abilities, his greatest threat is only human. Perfectly matched so that neither can win, given both of them playing the rules. Same for Batman/Joker
He’s only a villain on Tuesdays. The rest of the week he’s a hero, an anti-hero, an anti-villain, but all the time he’s a royal pain in the arse. He’s James Jesse, the first, the one and the only, the Trickster!
Conceptually, it’s Brainiac. So alien, so difficult to divine his motives. In truth, it has to be a well written Darkseid. There is something about a being as powerful as Darkseid who still feels such needs to control others that fascinates me.
I like Dr. Sivana because he’s a mad, mad scientist with a history of incredible feats (new elements named after himself, his own villain family, intangibility via math). If there is a villain parallel to Silver Age Superman, it’s Sivana.