Those being of course...
Fistful of Dollars
For a Few Dollars More
The Good, The Bad, and the Ugly
Once Upon a Time In the West
Duck You Sucker (AKA Fistful of Dynamite)
I always struggle with this ranking because I feel all of them are A level films.
I think I'd have to go with
1. TGTBATU- This movie is a good mix of the earlier fun Dollars romps and the more somber later films. It's almost a perfect movie. The civil war scenes are pretty touching stuff to put into the action framework, but it meshes perfectly.
2. For a Few Dollars More- Just a perfect fun western. I prefer Lee Van Cleef in this film over his later performance as "The Bad". These movies got more epic in scope and dramatic with each outing, This may have been the high point of the simple adventure style.
3. Duck You Sucker - Yep, I rank this one more highly than most. It's become the forgotten Leone film, But I think it's among his best. The music is beautiful. Sean and Juan's relationship is very touching. I think the political nature of the film turns off some people (especially because it's so cynical) but it feels very true to life for me. It should be noted the explosions are amazing.The bridge explosion here even tops the one seen in TGTBATU. The soundtrack laid over that scene show the brillance of Leone and Morricone, playing it for high emotion rather than straight action.
4. OUATITW - This is Leone's most technically accomplished film. It can be a bit lumbering, but I wouldn't have it any other way. It's just beautiful to watch.The film is almost an opera. I almost always tear up a bit at the ending, with the railroad finished and Jill bringing water to the workers. It's great that he got to shoot in Monument Valley.
5. Fistful of Dollars - Again it shows how great Leone's Western were that This is in last place, a film that is among the finest westerns ever made. It makes great use of it's small budget and feels so much bigger than it actually is. It only suffers in comparison for me due to what Leone was able to accomplish in later films.
As a bonus, I think my favorite western of all time is Shane.