Blade Runner gets better the more you watch it. I thought it was "meh" when I first saw it - now it's my favorite film of all time.
Lawrence of Arabia simply cannot be appreciated on a TV screen. You need to see it in a movie theater, with the largest screen possible, preferably in 70mm (which is how it was shot). It is a completely different experience, in such a good way.
2001 is, indeed, long and boring. That movie should only have been about an hour and forty minutes, not two hours twenty. I like 2010 (yes, with Roy Scheider) much, MUCH better than 2001. Having said that, 2001 is an absolutely perfect film if you watch it while on LSD.
Easy Rider isn't terrible, but it is grossly overrated.
It's a Wonderful Life - oh dear God how much I hate this movie. I'm so glad I'm not alone in that.
Saving Private Ryan - the first 45 minutes or so are good, the rest is pretty terrible. Band of Brothers is a far better WWII-related film, despite the fact that it's a television miniseries and not a film at all.
Zoolander is just terrible.
Now for my own additions ... hmm ... firstly, Vertigo. I don't hate it, I just don't get why it's supposedly such a "masterpiece."
Apocalypse Now Redux. Completely ruins the power of the theatrical cut, yet I know people who've only seen the Redux version.
A Clockwork Orange. I absolutely love the book. The film? It made me want to hunt down and destroy every copy of it, anywhere.
Full Metal Jacket. Just not good after the boot camp part.
Braveheart. Piece of shit film.
Slumdog Millionaire. See Braveheart.
That's all I've got right now.