Great topic, but I'm never too good at ranking.
OUATITW is probably my favorite. It's also the one I've watched most recently. The scale, ambition and masterful confidence in every part of it always impresses me. Before I saw it many years ago, I resented the idea of a Leone Western without Eastwood, but I never missed him once the movie started. Good old Henry Fonda as Frank, unforgettable.
I like TGTBATU and FAFDM pretty close to equal, too. I like the sprawl of TGTB and it has some of the greatest character moments of all, but sometimes the historical stuff seems too "off" for me personally and work against it. "A Few Dollars More" is great fun and I agree Van Cleef is at the top of his form.
"Fistful" is the one that got the ball rolling and was such an influence in so many ways. It was also the first one I remember seeing on TV as a little kid. Some wonderful tension-building. A great movie I'd watch almost any time.
I also grew up knowing the name "A Fistful of Dynamte." I knew the line "Duck you sucker!" mostly from being sampled in a song by Big Audio Dynamite. I haven't seen that movie in a long time and only once. It was good but I don't remember enough to make a worthwhile comment.
As a bonus, I think my favorite western of all time is Shane.
Wonderful. It would certainly be one of mine, along with Fort Apache, The Searchers, The Ox-Bow Incident, Red River
and... more. Again, I'm no good at ranking. I have to say, though, as great as Leone is, his Westerns never hit me on the same level as those of Ford, Mann, Peckinpah or Boetticher. I feel like as a European he has a distance from the material, which works in its own way, but not as much for me personally.