Which of these diets should I go on?

I am having a bit of a weight problem and I do not know what to do since I lost a ton of weight the first time without trying. I am an online college student and sit most of the time.I am 180 pounds. 5'5.Should I go on the South Beach Diet, Atkins, Special K, Slim Fast 321, or something else?suggestions are welcome ..and thanks guys in advance