Tombstone is of course great. Definitely one of the GOATS.
The good the bad and the ugly - I mean, hard to beat that one. The other spaghetti westerns with Eastwood are also excellent.
Silverado (80's) is really good.
Unforgiven of course
3:10 to Yuma was very good.
Django is great - hard to not watch it if it is on.
Eastwood has so many 70's era great westerns, it's hard to remember them all. High Plains Drifter. The Outlaw Josey Wales. And a bunch more.
I know I am forgetting some that deserve a mention here.
EDIT: Young Guns needs a mention. Also Paint Your Wagon with Clint and Lee Marvin is excellent. Can't beat the song they play when the movie ends.