I personally cast my vote for The Searchers. I think other westerns lack the psychological element that brought this film to another level for me. I also think John Wayne's performance was stellar. In addition, there were some really great shots throughout the movie.
I also enjoy Sergio Leone's spaghetti westerns which I thought have pretty cool action scenes and all, but I still think The Searchers is on another tier, even when also compared with Ford's other works.