Realistic melee: Oldboy, hammer scene. Theres nothing fancy about the fight choreography. It's just a one-against-all brawl in a corridor. With a hammer.
Absurd melee: The Spirit, fight between The Spirit and Octopus. It's just hilarious to watch. It's the closest thing you can get when you cross Sin City and the Looney Toons.
Realistic guns: In Bruges, ending scene. It's not much of a gunfight to be honest, but it has the bonus of seeming real. There's nothing about it that seems too over the top, theres alot of plot thats led up to the scene and it has genuine emotional ties leading to it. Unlike alot of other movies who have fight scenes for no other reason than "we need people dead", this scene had an actual reason to exist. Brilliant movie.
Absurd guns: Shoot 'Em Up, starting from the introduction of the carrot to the 'tit for tat' dialogue. From the moment you see someone get stabbed through the throat with a carrot you know what kind of movie you're watching. Absolutely brilliant at giving the target audience exactly what they want. Absolutely sublime comedic hyper violence.