I can't really decide, but my votes are
1. Curing LR . . . my girlfriend and I stayed in a hotel it was that bad, good thing it only lasted 3 days.
2. A dead DSB . . . When moving my tank, I drained the water out of the old and scooped as much of the LS out as I could, but there was still about an inch or so left on the bottom. After sitting out for a day I could barely stomach the scent.