When I saw this thread over at IMDb, Heat was the first movie that came to mind. Glad to see it at number 1. This is a shootout that several critics said was too over the top to be believable, at the time of the films release. Yet, within a year, Los Angeles police had to cope with such an ordeal, with two machine gun toting bank robbers that held them at bay for quite a long time, and played out live on the news.
I, too, had thought of the discotheque scene from Mann's Collateral. He just has a very keen eye for this type of action... riveting up the tension as the scenes progress. Two other films of his that I haven't seen mentioned in discussion are Thief and Manhunter, both of which feature good, tense shootouts at the end, although not on the scale of the films mentioned in your list.
One shootout I would have considered occurs towards the middle of John Carpenter's Assault on Precinct 13, at the point where the hoods are making their initial assault on the station house, and the deputy in command has to make the decision to not only release prisoners from their cells, but also arm them to help in the defense of the station. Carpenter, in his early career, also had a way of ratcheting up the tension as a scene progressed, rather than peak an action sequence too soon and thus run the scene past the point of believabilty and tolerance.