South Beach Diet
Lost ten pounds in two weeks, but after the first week or so of very few carbs I found I barely had the energy to walk up the hill from work to home.
Also, after three or four weeks I realized that I was only eating 1200 calories a day (which is not enough for a 5'10 woman who is exercising an hour a day six days a week).
It's not because I wasn't hungry but because I found my food options unappealing. (As in...I can't stand to eat one more bite of baked chicken breast or low-fat cheese or cut-up veggies.)
Gained all the weight back and then some.