Hrmm... Interesting question. I have to go with the double-barreled shotgun, although I hear Bulletstorm uses an all-new four-barreled shotgun, so there's a new contender in the race.
Also a python .357 revovler... just beautifully slender to look at, but also so powerful with those clips; simply huge for a hundgun.
In fact as far as revolvers go in video games, Ray McCall's dual pistols in Call of Juarez Bound in Blood which reflect the enviroment with their shiny metal are impressive as hell.
But honestly... it could only be more awesome if it had tits and was on fire: that right, it's the weapons of Painkiller (and Bulletstorm) which are the coolest, simply because the originality makes one marvel at them. I mean, come on! A grenade launcher which folds out into a minigun?! (Oops completely forgot TF2 there... which acctually has a combination of a revolver and a grenade launcher. Hmm, food for thought... [man, you ask a tough question]).
(Yay, post number 3. I really have to stop with this now.)
Edit: I find it ironic that the L85A2 is getting so many mentions the day after it was given as a gift to the players of the new F2P on steam (A.V.A.). It's almost too much of a coincidence...