Westerns in no particular order....

Little Big Man  One of my favorites....hilarious.   Makes Custer out to be the Vagina that he was.
http://www.imdb.com/title/tt0065988/

Butch Cassidy and the Sundance Kid   Absolute must see
http://www.imdb.com/title/tt0064115/

The Wild Bunch    When this movie was made it was way over the top.  It it wasn't for this film, westerns would still be benign
http://www.imdb.com/title/tt0065214/

How the west was won   Sort of hokey, one of the longer, epic tales of the west.   All star cast.
http://www.imdb.com/title/tt0056085/

The searchers   Considered by many people to be the best western of all time, in my top 5.  Another epic.  The one John Wayne film that you have to watch.
http://www.imdb.com/title/tt0049730/

Shane         Shane!  Come back Shane!!  What you would expect from a 50's western.
http://www.imdb.com/title/tt0046303/

Red River      One of the better JW flicks.  
http://www.imdb.com/title/tt0040724/

Stagecoach     An oldie but a goodie.  Made in 1939.  This film made Wayne a superstar.
http://www.imdb.com/title/tt0031971/

The good, the bad, the ugly      Although Clint Eastwood was on his way, this movie really put him on the map.
http://www.imdb.com/title/tt0060196/

Unforgiven     When I saw this in the theater through my teenage eyes, I hated this movie, Now it is my absolute favorite.  Watch this one first.  The genre was dead, this put new life into the western.
http://www.imdb.com/title/tt0105695/

The Life and Times of Judge Roy Bean
http://www.imdb.com/title/tt0068853/

There is a but-load of others, will finish this list soon.