Here is my list, I'm using IMDB as I know it is probably the greatest and most mainstream forum for film discussion. I often post and vote/review on the site. So here is my list.
To start. The Shawshank Redemption. Man I love this movie, Tim Robbins is good, so is Morgan Freeman. But the greatest film ever made? I don't think so. I'm not sure I'd even name it top 10, yet its #1 on IMDB.
Probably the most recent film I'll list here. The Dark Knight. Man I have never seen a movie that has gotten so overrated. Heath Ledger gave an amazing performance undoubtedly, but that does not put off its problems. Too long, weak plot, weak characters including Bale's Batman.
The Empire Strikes Back. Man I don't get why this one is rated so high, am I the only one who thinks a New Hope was the best Star Wars film? Maybe it's just me...
City of God. Another film I love and it was a breath of fresh air. But among the 20 greatest films ever made? I don't think so...
Fight Club. I think this movie has one thing going for it. Its cool. And Brad Pitt is cool. And the companies blowing up is cool. But honestly, its not that great folks!
Se7en. I mean, it was a good movie, it was interesting, and you know what else it was, completely predictable! I still don't get what's so great about this movie! I like it but give me a break it isn't one of the greatest ever made.
Leon aka the Professional. This movie is so incrediably shitty I can't believe its ranked so high. Natalie Portman is annoying. Leon just seems like a weirdo pedophile. Little to no character development. Shit dialogue. Stupid villains. Dumb plot.
American History X. Honestly I think people just like this movie so they can hear ******! and KIKE! and other cuss words, as well as seeing a guy get his jaw knocked in half. Beyond that its predictable with weak characters and a weak and predictable plotline. How this gets on a top 250, and even then top 50 is beyond reason.
Amelie. Okay its a cute French film interesting dialogue and story but is it really among the top 50 films ever made? I don't think so.
Psycho. Oh my God, so scary! Not. So overrated.
Here are the top 10 films NOT in IMDB's top 50 that I think should be.
#1. Breathless (Jean-Luc Godard)[not listed in 250]
#2. Raging Bull (Scorsese)[71 in 250 (wtf?)]
#3. Tokyo Story (Yasujiro) [not listed]
#4. Band of Outsiders (Jean-Luc Godard)[not listed]
#5. Blade Runner (Scott)
#6. La Dolce Vita (Fellini) 
#7. Il Postino (Radford) [not listed, severely underrated]
#8. Kingdom of Heaven (Scott)[not listed, severely underrated]
#9. Children of Men (Cuaron) 
#10. Patton (Schaffner) [208, lol what a joke]