I would say that living in Texas for most of my life has made me quite open to his films.
Ranking them, however, is the hard part.
First, I have to exclude Once Upon a Time in America, as I have yet to see it.
But then, I guess it would be like this:
1. Once Upon a Time in the West
2. The Good, The Bad, and the Ugly
3. A Fistful of Dynamite/Duck, you Sucker
4. For a Few Dollars More
5. A Fistful of Dollars
My brother and I watched FaFDM the other night, and we both ended up agreeing that it is better than FoD. Sure, the first has the nostalgia value, but I find FDM to have the better story and presentation. Also, it seems to have an especially eerie quality to it which I find appealing.