Hmmm ... Can't pick just 5 but here are some of my favorites
Requiem for a Dream - AWESOME acting and directing. Ellen Burnstein's performance as Sara Goldfarb was some of the most convincing acting I have ever seen, and to think the lady was 68 years old at the time. Marlan Wayans, Jared Leto, and Jennifer Connely also did tremendous Jobs. Aranovsky made this movie Even better than Pi, which is also one of my favorites. A very emotional movie, Even though im not into that style of music, The soundtrack was very very well written as well. This movie scared the hell out of me and really made me rethink life.
Braveheart - A really a Great Epic film, perhaps a bit of an ego trip for Gibson, but still a very woving story
Matrix - Not really a big fan of Keanu Reeves, but this movie is just great, not getting into details since everybody has already seen it.
Trainspotting - This has some serious entertainment Value
Office Space - One funny ass movie Period
Gattaca - a bit slow, not much action, But I think this movie has a lot of meaning
The Fifth Element - IMO one of Bruce Willis best films (except maybe the first and second Die Hard movies) Very entertaining, cool effects, decent plot. another plus - Milla jovovich.
South Park - I guess I'm just a big fan of Matt and Trey's work. Offensive, and funny as hell.
Many others were great (some mentioned already by fellow anandteckers .. Fight Club, Forest Gump, Armageddon, The Patriot, so many more ...) Movies rule