I don't buy that except in the most literal sense. yes, of course, the worst film ever is probably some badly made, no-budget, no script piece of shit that most of us will never even hear about, let alone see. I bet nobody here has ever seen the worst film ever by that criteria.
So what actually ends up getting listed is either big films that were shit, films that fell short of their ambitions or were missed opportunities or big films that were mystifyingly well-regarded when they're clearly absolute toilet.
Hence American Beauty.
Feel free to have a 'worst film' thread by your criteria, but i doubt anyone of us would even be able to post on it