"Raiders of the Lost Ark", Indiana Jones vs. the sword guy! It was actually an ad lib by Harrison Ford and not in the script but they kept it in. Already posted but what a great off-the-cuff inspiration from Harrison.
Dirty Harry's first shootout, at the bank robbery, comes to mind. And of course the whole street shootout in Heat, which probably has the most realistic gunfire sounds ever put in a movie. It rocked my TV--must have been deafening in a theater!
One of my very favorite scenes of all time is in the original John Carpenter Assault on Precinct 13 where at the last possible second, Bishop (the cop) manages to bust the pump shotgun out of its box and toss it to prisoner Napoleon Wilson, who, in one fluid motion, catches it, spins, pumps, and cranks off five quick shots. The action starts at about 1:20 in this clip:
In a somewhat similar vein, I love the whole hotel shootout in the original The Getaway, but particularly the moment when they're walking down the hall and McQueen spins around with the Hi-Standard pump shotgun, raising it above Ali McGraw's head as he comes through her, then lowers it and blasts a gun thug. Ah, the good old days, when movie actors actually knew how to handle firearms! (Honorable mention: The scene right after the bank robbery where McQueen lights up Rudy, point-shooting and left-handed. Someday I'll be that good.)