Worst movie ever? I can't narrow it down to one particular movie, as I've seen a lot and there are some awful ones out there, and some of them make a mint. Most remakes and adaptations of tv series are pretty bad, and I've not seen a really good adaptation of a video game to the big screen yet.
Good example, the Halloween remakes by Rob Zombie. Just... WHY???
Avatar - not bad, but not particularly good either. It's a rehashed idea that's been done and done better in other movies, notably
Dances with Wolves (the Dances with Smurfs episode of South Park is priceless) and even
The Last Samurai. The big draw for that movie was the special effects, which I'll admit were spectacular, but didn't do enough to compensate for the horror that was most of the acting in that movie.
But if you want the worst of the worst, pick up an eighties era slasher flick. Ugh.
Here's a pretty good spoof of that whole genre.