I'm surprised no one made this after seeing all the fans in the dead directors thread. He's one of my favorite directors and an excellent one. He's also the only director I know who has a trilogy that got exponentially better with each film rather then the first being the best.
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Best Films (Not Favorite)

1) Once Upon a Time in America, a breaktaking piece of filmmaking, one of the ten best ever IMO. Along with Raging Bull it's also the best of the 80's. Leone brings an amazing amount of depth to a gangster film. Every frame is excellent and screams brilliance. From Morricone's music to Leone's direction to the great cast (including younger cast), I give this ****

2) Once Upon a Time in the West is one of the most operatic films ever, and one of the few (if not only) opera art westerns. Many people considered to be "Tedium in the Tumbleweeds" upon it's first butchered release but in years to come it would receive high acclaim from inspired directors. Another delight is the excellent score from Morricone for each character, which would inspire future films. The cinematography is wonderful as usual, and the beginning is pure Leone. Rather then music, 11 minutes of just about no dialogue, only action. My Review - ****

3) The Good, The Bad, and The Ugly lacks the depth of #2 on this list, but it's another masterpiece of the western genre. Morricone's music goes on to be a pop-culture icon itself. The story is well formed, and well chosen in it's actors Clint Eastwood, Lee Van Cleef, and Eli Wallach. A grand epic that all leads to one of two or three best stand-offs in film history. Leone directs it with his slow panning, absorbing the wonderfully gritty cinematography.****

4) For a Few Dollars More is an improvement on #5. It's story improves as well as the direction, which has inspired many future films and other spaghetti westerns, such as Death Rides a Horse. Clint Eastwood stars opposite Lee Van Cleef (as a good guy this time) in this tale of revenge in the world of bounty hunters. The cinematography is once again great, as well as the memorable scenes. Morricone once again provides a wonderful score. ***1/2

5) A Fistful of Dollars might not be the best or the first spaghetti western, but it did three things. Popularize the sub-genre, help in Clint Eastwood's rise to the public light, and bring Leone and Morricone a step up from past low-budget projects. Leone brings a very good direction, combined with Morricone's great score. Clint Eastwood's character quickly becomes the cult icon it still is today. This also features Gian Maria Volonte in a nice role as the villain. (same can be said for #4) - ***1/2

Leone's dream was supposedly making another adaptation of Gone With the Wind. I would've loved to see his version, not because I think it would surpass the classic 1939 version, but just to see how he would handle it. I would love to see that slow panning camera go over Tara and Twelve Oaks.