That is an excellent question.
I think the worst park day (or partial day) I ever experienced was at Six Flags America (of all places) a few years ago. I stopped there with some friends on opening day while en route to Great Adventure. It took forever to get one guy's pass processed, and he needed every credit in the park. Well, he got Wild One--that's it. To top it off, one of our group was evaced off of Joker's Jinx, and park security and local cops had to drag some pervert out of the Superman line (he was groping teenage girls).
To be fair, this is the only "bad" day I've had at that park, but I haven't been back since.