Always get it hard to pick out one particular film from many that I like so i thought I'd just list some of my favourites in no order -
Jean de Florette/Manon des Scource - Great story and incredible acting from the main characters Montant, Depardieu, Auteil and Beart
Casablanca - what can I say, great main story and intriguing smaller ones
Treasure of the Sierra Madre - Bogie's best work
Twelve Angry Men - 12 men and one room and a murder
Once Upon a Time in the West - great intro sequence, superb music and Henry Fonda in an unexpected role
Downfall - last days of Hitler, well acted especially Bruno Ganz
Godfather 1,2,3 - three of the best actors of all time, giving their best DeNiro, Brando and Pacino
and where do the LOTR films fit into this, I loved them , like the book however, they weren't long enough;-)