Heat, after the botched bank heist. Greatest fucking shootout in the history of shootouts, I say. I was reading trivia about that scene, apparently more rounds were fired for that one scene than in any other movie
A close second is in Collateral, during the nightclub scene when cruise's character is taking on the entire world with nothing more than a pocket knife and a .45 USP. I believe he said it best after making his getaway, "the only thing that didn't show up was the Polish calvary"
In case you couldn't tell, I'm a big fan of Michael Mann's movies. Another movie with badass gunfights is John Woo's "Once a Thief". Holy shit, one of the greatest movies to come out of Hong Kong
