Best
1. 12 Monkeys (Bruce Willis and Brad Pitt in a time travel movie directed by Terry Gilliam, that actually manages to pull off the closed loop without becoming a paradox...something which Terminator has yet to do)
2. Snatch (most mentioned, would win the thread if anybody's counting)
http://www.youtube.com/watch?v=kDKiQfBs9lo
3. Goodfellas (how am I the first one to say this? For shame, Escapist, for shame)
4. Natural Born Killers (any movie in which Woody Harrelson is awesome is just a pale shadow of the demonic force of nature that is Mickey. The balls to make a movie starring two serial killers, with Rodney Dangerfield as an incestuous father. Awesome soundtrack too)
5. Dark City (imagine the Matrix, but it's all physically real. No AI, just dying aliens as parasites in our dead. Throw in a profound plot about how unique the Self really is, an oddball cameo by Keifer Sutherland, and one of the most surprising scenes you will ever see. You'll know it when you see it.)
If the list could keep going: Dr. Strangelove, Braveheart, The Thing, The Professional(mostly because of Gary Oldman), Road Warrior, Gangs of New York... Notice Fallout wouldn't be possible without two of them.
I didn't say Fight Club because Chuck Palhniuk has books that would make MUCH better movies. Haunted, Lullaby, Rant...
Worst
1. Eraserhead (it's meant to hurt you, and it will. This is nightmare rocket fuel. I don't care how much you like horror, this movie is impossible to enjoy on any level.)
2. Manos, the Hand of Fate (barely qualifies as a movie)
3. The Incredibly Strange Creatures Who Stopped Living and Became Mixed-Up Zombies (you know I hate it if I bothered to type that out. Only movie that's ever made me nauseous.)
4. Hobgoblins (you WILL want to personally murder every character.)
5. Monster-A-Go-Go (THERE WAS NO MONSTER!)
EDIT: Worst Intent behind a movie- Ben Stein's Expelled. Whenever I see an ad that uses him, I make a mental note to never give that company money.