By Genre,
Drama: "Amadeus" - It has the best villian I have ever seen, although that very much depends on how you like your villians I guess. Even then it is still a great movie.
"A Man For All Seasons"; great movie about Sir Thomas Moore, based on a play.
Roman Polanski's "Macbeth". Sick SOB Roman Polanski is a great choice to direct a movie about Sick SOB Macbeth. This qualifies as "horror" too depending on your definition of that genre.
"Cool Hand Luke": Best prison movie ever! (yes, better than "The Shawshank Redemption")
Action: "The Last Action Hero" (if you ignore the stupid kid, its pretty good) and seconding "Total Recall".
Seconding "Heat" if only for the bank heist scene, and "Ronin" is good if only for its car chase.
"Robocop": only get the first one though, Robocop 2 and 3 are not good.
Adventure: "The Man Who Would be King" (those who say "Raiders of the Lost Ark" is the best adventure film have probably not seen this)
"Last of the Mohicans"- overlaps with "war" somewhat. A good movie taking place in the French and Indian War.
Thriller: "Spy Game"
Horror: "The Golem", "Faust" (classic horror) If you meant slasher flicks like "What Sort of Sicko Likes Seeing Attractive Girls Gruesomely Killed?: Cheerleader Babysitter Slumber Party Camp Massacre #7287" I Hate those, and honesltly think anyone who likes them ain't right in the head. "Evil Dead 2" and "Army of Darkness" are good horromedy though, although Army of Darkness is much, much more comedy.
Western: "The Good, the Bad, and the Ugly" is quite good. "The Searchers" is also very good, although if you are PC you might hate it. "The Outlaw Joesey Wales" is also goood.
Also "Tombstone" is good just for the chareacter of Doc Holliday, I forget the actor's name but he was Ice Man in "Top Gun".
Sci Fi: Seconding "Blade Runner".
"A Trip to the Moon": one of the first and still one of the best.
"12 Monkeys" -good time travel film.
Fantasy: "Excalibur"- best King Arthur movie yet.
"The Princess Bride". People will think you're a few bricks shy of a load if you don't like this movie.
"Labrynth"- Awesome Muppetry! And I will question your manhood and/or sanity if a young Jennifer Connelly doesn't appeal to you. I didn't really like David Bowie in it though.
"Apocalyse Now" (No, its not a war movie). Its based much more on a book than a war, but its still entertaining.
War: "All Quiet on the Western Front". The scene of a Maxim gun mowing down Frecnch troops is one of the best things put to fiilm.
If you get "Saving Private Ryan", only the opening Normandy battle scene is good, otherwise, as someone else put it, the rest is Germans acting "like Indians in a bad Western".
"A Bridge Too Far"- is pretty good, one of the best war movies IMHO.
"Gettysburg": Awesome movie based on the awesome novel The Killer Angels. Has a good soundtrack too. only real problems are the lack of blood and the unhistorically plump re-enactors, especially the Confederates.
"Zulu": Its basically "Black Hawk Down:1879".
"12 O'Clock High" pretty good movie about the early American bomber operations over Europe.
"Ich War Neunzehn"(I was nineteen) : Great movie about the last days of Nazi Germany. It was made in East Germany in the 60s, but it still has a less friendly portrayal of the Soviets than the recent "Downfall" movie. It has an atmosphere that "The Downfall" just can't match, it actually seems like the last days of a war while "Downfall" seems like a melodramtic stage play. And it actually had a much friendlier portrayal of Germans than "The Downfall".
"Gallipoli"- a suprisingly accurate movie about the ANZAC attack in the Dardanelles in 1915. the best movie ever involving Mel Gibson, and the only movie I've seen where what I call the "European Ending" works.
Romantic Comedy: "Groundhog's Dog". Bill Murray is awesome! "Grosse Point Blank" also is OK.
Crime: "The Sting" :excellent and funny.
"Ocean's Eleven" -the original, not the re-make.
Comedy: "Wayne's World". Mike Meyer's at his best.
Seconding "Dr. Stangelove".
"The Producers": "Springtime for Hitler and Germany....winter for Poland and France"= ROFLMAO!!! Best musical number ever!