90s- Unforgiven
80s- I'm not aware of many great 80s westerns, so I'd say Pale Rider.
70s- The Outlaw Josey Wales, and I love McCabe and Mrs. Miller
60s- Once Upon a Time in the West, The Dollars Trilogy, The Wild Bunch
50s- The Searchers (Not just one of the best westerns, but one of the best American movies to ever be made)
40s- The Oxbow Incident, Treasure of the Sierra Madre
30s- Stagecoach
I think you have a good string here leading back to early films. If you follow the strand of Eastwood through Unforgiven to Pale Rider to The Outlaw Josey Wales to the Dollars Trilogy, you'll notice a very interesting evolution of the Western protagonist. Pair that with the evolution of Henry Fonda from The Oxbow Incident to Once Upon a Time in the West, and the stereotyped protagonists of Stagecoach through The Searchers and I think you'll find some interesting things in some Great films.