FN FAL w/ Masterkey from MW2. Even more so with an ACOG. Though I don't remember if that's possible.

Or the pistol from Halo 1.

Can't think of anything else that might make the cut, my selection of video games is fairly slim.

The OICW from Ghost Recon 2.
Nothing was cooler than holding it over some cover and shooting some really surprised enemies.

After that, the "DD44 Dostovei" from Goldeneye N64. Probably one of the only Tokarevs in a non-WW2 game, and certainly one of the most powerful portrayals of it in a game.