Cowboys and Aliens
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Stands on its own as a good hybrid of sci-fi and Western.
The characters start off as the typical Western tropes; silent and mysterious stranger with a past, cattle barons who think they run the town, worn down sheriff trying to keep the peace.
As the action progresses they realise they all have things that will help the others, and they become a lot more sympathetic than if this was just a run of the mill Western where good and bad is often shoved in your face.