I don't think I could narrow it down to just one, I'd have to have some different categories at least.
For an overall classic movie scene, definitely Clint Eastwood. My all time favorite is the "make my day" scene in the diner in Sudden Impact, I pull out the DVD every few months just to watch it. There are some more really classic scenes from his movies: Just off the top of my head, the end of A Fistful of Dollars with the iron plate as body armor and rifle vs. pistol contest, the ending battle of Unforgiven that spamanon mentioned, probably plenty more I can't recall.
For a modern war movie, pretty much any and all shooting in Black Hawk Down. Older war movies are a tougher category, but I think the most memorable is from Patton, where he gets riled up and goes out into the street to shoot at German planes with his .45 and miraculously drives them off.
Then there's flashier special effects and weapon showcase type movies. The lobby shootout and subsequent fighting from the Matrix is pretty much a given. The Underworld movies are great at showing off weapons, especially the P99s and full-auto-converted Berettas that Selene carries.