My favourites:
Sergio Leone's "Dollars Trilogy" with Clint Eastwood -
"A Fistful of Dollars" (effectively a remake of Akira Kurosawa's classic "Yojimbo")
"For a Few Dollars More"
"The Good, the Bad, and the Ugly"
Clint Eastwood's Oscar winning classic from 1992, "Unforgiven"
Sam Peckinpah's excellent 1969 Western, "The Wild Bunch"
John Sturges' "The Magnificent Seven" (a remake of Kurosawa's excellent "Seven Samurai")
EDIT:
I also quite like "High Plains Drifter", "The Outlaw Josey Wales", and "Pale Rider"... all Clint Eastwood vehicles.