IT's the kooky assassin with a death wish, DEADSHOT, if I'm correct he's also the first DC villain to warrant a second mini-series under his own name. I'm also quite fond of Captain Boomerang, of course, he's a loser, but he killed and threw off a tower a guy who made fun of him. That was pretty cool, plus he's an untrustworthy, backstabbing opportunist (sending Slipknot away to see whether his arm would really be blown off or throwing pies in Belle Reve). I wonder why he's so fat in Identity Crisis now, but it seems he'll have a larger role to play there. Still, Floyd is so much cooler, hence my username.
Then there's Lord Naga Naga, or Naja Naja, a.k.a. Kobra. I like how he just squanders his underlings lives.
On the Marvel side I like Thanos best. Classic line: Surely you jest!