Depends on precisely what you mean by worst.

There is a deep, objective sense in which spending more than a year on a site that never got launched was bad. A permanent loss of meaning for that time. But it doesn't hit quite so hard as, for instance, being woken up every night for two weeks due to a bug I wasn't allowed to fix in an integration with our monitoring system, or when the amped-on-amphetimines founder threw a chair at the guy I was sharing an office with (and later explained how this was a better way to handle their disagreement than firing him, because he was really quite talented, wouldn't firing him have been bad for the company?) -- this made it impossible to work, the anxiety was just....

I think that's the odd part. A lot of the answers here have the distance of time, and know what's important. At the time, stuff like that, like having the management team demean and laugh at you for being concerned about honesty with business partners -- it has such a salient emotional impact it's hard to remember it's not even in the right category for 'worst thing'.