I can't really decide between these four:

Vash the Stampede (Trigun): Unbelievable skill and power, though you really don't see it often enough due to his demeanor.

Rushuna Tendo (Grenadier): Pretty much the female version of Vash, but with a better reloading method. Heh. Without a doubt, the most rediculous, over the top gunslinger. Her skills are just obscene.

Rally Vincent (Gunsmith Cats): Excellent marksmanship skills, and she shoots off people's trigger fingers. She knows her guns and cars better than anyone. Most realistic one too. I always thought Minnie-May was better, but this thread isn't about explosives.

Train Heartnet (Black Cat): Though the series is still in its infancy, if you're a reader of the manga, you know how skilled he is. Unbelievable accuracy, quick on the draw, and some other skills that would count as spoilers so I'll leave out.

Brandon Heat has nothing on these four.