I really hated the movie Heat, but the opening and the famous "fire & movement" scene were undeniably sweet.
Michael Mann did both Heat and Collateral, which I really liked. The "hey homeboy" scene was short and sweet CCW action.
Before reading any of the responses in this thread I stopped and thought about my answer, and Way of the Gun immediately popped into my head as far as believable realism. One of my favorite movies to watch with the director commentary on.
Leon the Professional had an absolutely brillant finale.
Saving Private Ryan was, of course, great, but no particular scene sticks out, except perhaps Tom Hanks unloading on the Tiger.
Enemy at the Gates, a must see for any Mosin Nagant fan, was fairly good throughout, but the scene where Vasily gets the rifle from the commisar and whacks the officers was my fave in the movie.
Ronin has some nice gunplay mixed with outstanding driving, but no one scene sticks out.
Resevoir Dogs and The Rocketeer both have great BG unloading with two 1911 scenes, but Last Man Standing pretty much beat that horse to death. I loved it, though.
Speaking of Harvey Keitel blasting at cops; I don't wish LEOs any ill-will at all, but there is something oddly satisfying about watching a unit getting lit-up. Of course that brings us back to Heat. Terminator 2 had an over-the-top orgy of Cop Car shooting gallery. The shotty in the flower box was nicely filmed, and the "spinning the lever action to cock it while riding the HD Fat-Boy" scene was pure Hollywood. (In a good way.)
Spaking of Hollywood over the top stuff, The Crow had a wonderful scene in the club where Brandon Lee soaks up a hail of gunfire.
The Bourne movies and the Lethal Weapon series had plenty of fun, and speaking of Mel Gibson, Payback was pretty nice, too.
Quigley, Last of the Mohicans, and The Patriot all had great period gunplay. There are so many good westerns out there, I can't even get started on them. Way of the Gun was intentionally done as a modern western.
I am a little surprised no one has mentioned Boondock Saints. I am REALLY surprised that no one has mentioned The Outlaw Josey Wales.
I understand Pearl Harbor was not well liked by all, but the control tower .50 and crash scene at the end were good fun.
In the end, it is a tough call for me between The Professional and Way of the Gun.