HEAT, by a mile
HEAT wins it for me.
It's pretty technically authentic (for Hollywood) and represents director Michael Mann's best work (although COLLATERAL and THIEF -- along with some of his old MIAMI VICE TV shows -- had some well-done scenes to go with those movies' gritty, anti-BlissNinny tone).
I think the thing that elevates HEAT is that it's a real running gun-battle, not just a static shoot-out... with both good-guys and bad-guys going down. Additionally, the whole bank-robbery sequence is "organic" in that it just flows naturally from story elements that build-up to this eruption of action. It's not a contrived excuse for an "action whammy" as so many action flicks are filled with.
Beyond that, HEAT's big bank-robbery shoot-out rang my bell because I spent 5 years working in office buildings right there... at the intersection of 6th and Flower in L.A... and walked that concrete/tarmac daily.
SCARFACE (the one with Al Pacino) has its moments (the finale), and THE REPLACEMENT KILLERS and THE PROFESSIONAL are pretty slick, too.
A SUGGESTION: Anyone interested in starting a similar thread for THE WORST movie gun-play? There's so much gun-dumb cinematic material to cover here, it's hard to know where to start -- but I'd point out the scene in THELMA & LOUISE... where the girls blow-up a tanker rig with a few shots from their .38 and .45 ACP. (Right. Hey, guess what: it IS a ChickFlick. Maybe that's why, as per other threads here, so many women hate/fear guns. Indoctrination...)