When you say shootout I assume you mean as in a gunfight with lots of lead flying around and not a classic duel where only one or two bullets are fired?
I am a huge Spaghetti Western fan and I really enjoyed the shoot out in "The Great Silence". It was just so...cool. The main character uses a Mauser Broom-handle C96 instead of your typical six shooter but still manages to draw his weapon from the stock/holster faster than the men with their SAAs. If you want to see a crazy Spaghetti Western check out a film called "Keoma", some really good shootouts in that one; it is a real man's man film. Another Spaghetti Film that would not be hard to find that has several good shootouts would be "Django". In the film Django drags around a coffin with a Gatling gun inside; very cool. Of course we have Sergio Leone's Spaghetti westerns, but I don't think I need to mention them; because who has not seen The Dollars Trilogy or Once Upon A Time In The West?
I would like to also mention "Red Dead Redemption", yes it is a video game
, but the story is every bit as good as some of the most famous Westerns.
On a side not, I believe the film "Reservoir Dogs" had the most realistic shootout I have ever seen. I know it is not a western per say, but it traces its roots back to Old West films and Tarantino is a huge Spaghetti Western fan too. This movie literally had me on edge after watching it. Something very few films have made me do. It just seemed so real that it was scary; but of course I know people who say it sucks, so each to their own.
I could go on and on, so I think its best I stop before I get too riled up.