Here's one scene from "The Warriors" that I found, but there are many more on youtube. In my opinion this is a very believable fight scene because the actual altercations or "fighting" is short and real. Even Jackie Chan in an interview described in a "real fight" it is usually over in about 30 seconds (no longer) and you are just totally exhausted and winded from the whole mental ordeal.
Some of the most violent fight scenes I've seen in a film are probably in "Drive", the film currently in theatres. I couldn't find clips on youtube, maybe once it's out on DVD.
