Are we talkin' gangster movies or any "must have" flicks? To the gangster genre, I'd add
Mean Streets,
Road to Perdition (yeah, I know it's not a "gangster" movie in the extreme sense of the term, but it's great),
Gotti,
Reservoir Dogs, and perhaps
The Untouchables (although I still haven't had the forethought to pick up the latter one yet).
Other flicks? IMHO:
Platoon, Patton, The Deer Hunter, Saving Pvt. Ryan, Tombstone, Excalibur, Braveheart, Schindler's List, Jesus of Nazareth (albeit I look eagerly forward to Mel Gibson's
The Passion due out next Easter),
LOTR I & II (please don't stone me for those!
), and, of course,
The Matrix.
