I've tried a bunch of them, but the one that's working for me is South Beach. Lost nearly 50 pounds since New Years (a resolution diet that's stuck). Shooting for another 20--I keep extending the goal as I progress.
My doc says that he thinks South Beach is the one that makes the most sense because you really can adopt it as a lifestyle rather than a diet. He's against Atkins because he says it's simply too much fat, and it "acts" like a diet and not a way of life.