I'll second Deadwood. It's a masterpiece.
Something I find quite amusing is that a lot of Westerns were inspired by or were adaptations of Samurai movies and those Samurai movies were inspired by pulp novels. Truly full circle.
Must see samurai movies (keeping this spoiler free)
Yojimbo - chances are you've seen one or both of the main western adaptations of this movie. Those being Fistful of Dollars and Django (1966) which is admittedly a bit of a looser adaptation. Toho actually sued over Fistful of Dollars. The sequel Sanjuro doesn't have as much of a "western" feel to it as it was an adaptation of another story where they then used the same character due to the popularity/success of Yojimbo. Desert Heat was a more modern adaptation of this movie. In any case it's great and timeless.
Seven Samurai - US version is the Magnificent Seven which pales in comparison imo.
The Samurai Trilogy - carve out some time for this one. Covers the life and deeds of Miyamoto Musashi spanning 5 hours across 3 movies.
The Sword of Doom - This is one of my all time favorites. Was going to be a trilogy but unfortunately that did not materialize.
Three Outlaw Samurai - a Samurai western through and through, I don't think it ever got any western adaptation but I really enjoyed this one.
Gate of Hell - one of my favorites from a lesser known director
Sansho the Bailiff - Not really a Samurai movie but it's a great movie that I always recommend. A powerful film.