1) Once Upon A Time In The West.
2) Nevada Smith.
3) The Magnificent Seven.
4) Shane.
5) The Good, The Bad, and the Ugly.
5-1/2) High Noon.

1.) Unforgiven
2.) The good, the bad, and the ugly
3.) quigley down under
4.) young guns
5.) young guns 2

I didn't really get into Westerns until the 90s, so my picks are not the "classics". Still have to see those.

1.) Tombstone
2.) Deadwood (Does a series count?)
3.) 3:10 to Yuma
4.) Lonesome Dove (Yet another series. . .)
5.) Young Guns