Buffalo wings at the Marriott Marquis (room service). Was sick for days, had to cancel Xmas plans, and fly home immediately. 8 pounds lost in 3 days.
Marrakech. Some higher-end place, too (all relative). Took the docs at Mass General who specialized in internal infectious diseases 4 weeks to tell me the parasite I contracted was so third world, they couldn't even identify it. That was good for 18 pounds over 4 weeks.
The Ritz - Newport Beach (NOT the RCL). Was so ill from eating salmon in the MIDDLE of dinner. Hit immediately, and lasted a good 12 hours. 2 pounds, 1 day.