I'm not sure where to start, maybe The Searchers with it's gray zone morals and John Wayne playing both sides of the line on morality? In any case, I know where I want to start
1) Once Upon a Time in The West
You know all bets are off in this huge and epic western when Henry Fonda shows up and you're used to those famous blues eyes being kind and heroic when Leone does one of the greatest swerves in cinematic history by revealing him as a stone cold killer and thief, like he's Richard Stark's Parker in the wild west. Just a classic and while I love the Dollars trilogy, this was always my favorite Leone.
You? I fully expect the usual suspects like The Wild Bunch to be in the top 10.