In the interest of diversity - I'll only mention a few that have yet to be covered (rather than rehash a few obviously great films)
SLC Punk - if you haven't seen this you are wrong. A refreshing change from modern hollywood with great monologue. Matt Lillard Rules.
Raising Cain - Scary....
Seven - Really scary ...
(above are in the freaky "too easy to be real" scary not the stupid monster that won't die horror silly scary)
JSA (Joint Security Area) - Korean film that does wonders for mytery / drama and originated the multi-lingual movies. Main Character is a Swiss - Korean - all dialog with Koreans is subtitled Korean, all dialog with non-Koreans is English (subtitled in Hangul for the Korean Market) Story is excellant and overall quality (save one really bad Swiss actor) is Hollywood grade or better. Available at Amazon.com in DVD all region.