They all work, if you stick with them, so that's the real trick. Find one that fits your personal tastes. The zone is tricky, as the others said, because you have to carefully measure out blocks of each protein, carbs, and fat for every single meal. You can't just eat what sounds good. South Beach is more lenient.
I would suggest looking at the approved food lists and some recipes for each plan and deciding which you think you can live with. Both the zone and south beach have lists of foods you can eat, and lots of foods you can't eat, unlike weight watchers. If you don't like what you are eating, you're more likely to go off your diet, so give it some thought before you make a choice