One that always comes to mind for me is the fight between Juliette Lewis and a guy in a diner in "Natural Born Killers". Granted, she is totally psychotic and is completely in the wrong for picking the fight with the guy in the first place...but from a "down and dirty" realistic fighting perspective, she was AWESOME! No fancy choreography, just total commitment as she nails him with an open palm strike to the face and knee strikes to the groin and head. It was pretty quick, but the whole time she's fighting, she yelling things like "How sexy am I now?" O.K., I said she was psychotic...
Also, the fight between John Cusack and Benny Urquidez is "Gross Point Blank" was great! They trained together in real life and it really showed. I'm sure they had a blast shooting the sequence!
And, of course, "Kill Bill" 1 & 2!