The Good, The Bad, The Ugly - this is a no brainer #1 for me, it's in my top 3 favorite movies of all time. Absolute masterpiece.

Unforgiven, Tombstone, Fistful of Dollars (a remake of Kurosawa's great Yojimbo since we're talking about Kurosawa), the Wild Bunch, The Searchers, Rio Bravo, and Stagecoach are also great.