I'm surprized that nobody else mentioned "A Fistfull of Dollars" until Buster dumped the IMDB laundry list on us.
The only thing that one was missing was Lee Van Cleef lol. I really thought it was the best of the Eastwood/Leone movies. I liked the premis of "A few Dollars more" a bit more, plus Lee Van Cleef was badass. But it ran a bit long, and started to lose some steam. I though that "The Good the Bad and the Ugly" became too cumbersome as the story went on. I really stopped caring after a while, which is a shame, as I really liked the first two movies.
Anyhow, my vote would go to "A fistfull of Dollars." I really should rent "Yojimo" sometime.
-Sik