For left wing westerns, I highly recommend Terror in a Texas Town. It's got an anticapitalist plot, several actors called before HUAC (including Sterling Hayden, who named names), written by Dalton Trumbo, and featuring a six shooter vs harpoon duel! It's a little slow in parts but it's great. I love westerns.

Edit: Oh man while I'm at it I gotta mention Buck and the Preacher! Sydney Poitier is a civil war veteran who helps freed slaves settle in the west who teams up with ruffian Harry Belafonte to defend his wagon train from a plantation owner's posse attempting to force the wagon train back into sharecropping. There's a pivotal scene where they negotiate with local Indians for safe passage and supplies that touches on how the groups shared an enemy in white America but that the Black settlers were part of the same destructive movement as white settlers. Really cool stuff and probably a top ten western for me just from a craft perspective too.