Depends a lot on the game, but usually the ones that look, sound and feel best. In Return to Castle Wolfenstein, the MG42 felt and sounded fantastic, and incidentally, it's predecessor MG3 in real life is also very, very fun. In Gears of War 3, definitely the sawed-off. I like assault rifles, but mostly when they allow for burst or semi firing, as you wouldn't fire it on full-auto in real life, on account of most assault rifles firing fast enough to empty a full 30-round clip in between 1 and 2 seconds. Yes, I've tried. Most modern military shooters also get the reloading times completely wrong. After one year in the army I could reload my assault rifle quite a bit faster than the supposedly highly trained soldiers in these games. I would also love it if a game actually implemented Norwegian versions of some rifles, like the HK416, and the AG3 (modified HK G3). A lot of the time I go for semi-auto or bolt-action rifles, like the M14, Mosin-Nagant and such, and again I would love to see a game implement a Krag-Jørgensen bolt-action.
But for pure, allround FUN, I picked pistols in the poll, because who doesn't love running around with akimbo Desert Eagles or Colt Pythons or what have you, it's just sweet.