A fistful of dollars is a classic western and I would highly recommend it. However, if you want to watch the original, you will have to look up Akira Kurasawa's Yojimbo.
Same plot, and it benefits from Kurasawa's masterful touch. This and The Magnificent Seven were both direct remakes of Kurasawa's films.
We won't mention the ripoff George Lucas did when he "created" the Star Wars trilogy. If you have any doubt, watch the "cantina" scene in Yojimbo.