Which diet causes the fastest weight loss? atkins? south beach?

i'm not trying to long-term lose weight. i just have 3 weeks before a vacation where i'll be in a bathing suit and i want to lose like 10-15 lb by then. (i'm 5'2" 120lb) as far as dieting, which diet causes the fastest weight loss? even if its unhealthy and comes right back lol. its only a week vacation. thanks!