(These are not in any particular order!)
1. The Joker
Okay, yes it is pretty cliche that the joker is on here, but how could he NOT be? He is completely unpredictable and just... insane! One moment he'll be trying to kill Batman, and the next he'll be saying how they're equals and that one couldn't live without the other. On top of that, lets just be honest, he is hilarious.
Not only is he all of that, but he is actually smart too! He can get out of Arkham whenever he tries to, he is manipulative, and he can actually nearly kill Batman (although we know he wont since he likes "playing" with Batman, also Batman is too tough to get killed by him.)
Abomination was AWESOME in The Incredible Hulk (2008). While he wasnt revealed until about 3/4 of theway through, he showed up with a bang! Abomination is a perfect match for the Hulk, they're both extremely strong and huge, but Hulk has more will to win.
Abomination in genuinely terrifying to be honest, although im not a fan of some looks of him in the comics. He knows what he is doing and can destroy just about anything due to his massive size and strength.
While Carnage is not as well-known or "buff" as Venom, i think he deserves the spot more (although i love both). Okay, Carnage is just... gross. Seriously. Just look at him! Carnage has the capability to take down Spider-Man, and that's pretty damn hard! Venom and Spider-Man had to team up. Do you know how hard it would be teaming up with Venom?! Give that some thought.
Clayface is pretty awesome ive got to say. he can turn into anyone or anything he wants! You know how awesome that'd be? It would sure be scary finding out someone you know was actually Clayface one day! Remember how it felt during the last fight in Batman: Arkham City when you thought it was the Joker, but it was Clayface the whole time? That sucked. But it rocked. Great plot-twist DC!!
Bane is one of those bad guys that is just... wow. He's awesome, he's tough, he's different, he's just Bane. I love every version of Bane (Yes, even the TDKR version; although not as much)! i honestly think my favorite version is from The Batman.
Bane's a guy that can destroy you. And you. And you. And literally everybody, except Batman, because... well... hes Batman. No one can defeat Batman.