I'd recommend Andrei Rublev. Pretty heavy going at the start but once it gets going it's phenomenal film-making on the Fitzcarraldo scale of epicness and sheer difficulty of production.
1. The Good, the Bad, and the Ugly (Sergio Leone)
2. Django (Sergio Corbucci)
3. Tokyo Drifter (Seijun Suzuki)
4. The Shooting (Monte Hellman)
5. Who's Afraid of Virginia Woolf? (Mike Nichols)
6. Persona (Ingmar Bergman)
7. Cul-de-sac (Roman Polański)
8. Andrei Rublev (Andrei Tarkovsky)
9. Seconds (John Frankenheimer)
10. Ride in the Whirlwind (Monte Hellman)
1. The Battle of Algiers (Gillo Pontecorvo)
2. Persona (Ingmar Bergman)
3. The Good the Bad and the Ugly (Sergio Leone)
4. Andrei Rublev (Andrei Tarkovsky)
5. Seconds (John Frankenheimer)
6. A Man for All Seasons (Fred Zinneman)
7. Tokyo Drifter (Seijun Suzuki)
8. El Dorado (Howard Hawks)
9. Cul De Sac (Roman Polanski)
10. Kill, Baby, Kill (Mario Bava)
There's a 5-way tie for 10th... I think I'm going to leave this game open-ended for now and just move on to another year, and then try and return to this one and hopefully I can get a few more lists.
Edit: Cheers to zerostatic for a last minute list.
1) The Good, The Bad, and The Ugly
2) El Dorado
3) The Ghost and Mr. Chicken
4) It's The Great Pumpkin, Charlie Brown
5) How the Grinch Stole Christmas!