Spaghetti westerns are great. I'm not sure I'd have ever truly gotten into westerns if I hadn't watched For a Few Dollars More on a random afternoon. After that I had to have more and boy was there some great stuff (And plenty I've still yet to see).
For non-Spaghetti, one definitely work mentioning is The Wild Bunch. Great movie about aged outlaws nearing the end of their rope. Long and violent with great performances all around.
While the story and characters are fantastic, one aspect that cannot be ignored is the action and execution of said action. The staging, photography and editing of the action scenes holds up even today, opening with an intense opening robbery shootout and ending with a legendary action fest involving a mounted machinegun. It's exciting, surprisingly gory(Basically squib-world) and I feel like this
had to have been a direct influence on Rambo '08's ending sequence. Said ending action sequence is worth watching the movie for alone, though you're getting a great movie with it, so you can't lose.