Hmmm tough one, i really like the Alien Pulse Gun but after watching Distraict 9 the other day i might be swaying towards there Electricity type rifles.

I like the seburo m10 from Ghost In The Shell: Solid State Society.

THE LANCER FROM GOW FTW! yes, the lancer all the way! :D

The Lancer is too big for me. I'd pick the Morita rifle from Starship Troopers.

The M41A Pulse Rifle from Aliens is one of my favorite fictional guns

The Cobra Assault Cannon from Robocop. The most fun you can have on a deserted street. Bit unweildy, though.

The cerebral bore from Turok :D

M-56 Smart Gun from Aliens

http://img707.imageshack.us/img707/8117/m56.jpg

Does that even qualify as fictional since its a largely unmodified Barret 82?

I'd say it fits, since it's using ammunition that doesn't exist in anywhere near what it's depicted as being.

I like the modified M60 machine gun used in Avatar. The ventilated handguard and the way they use a drum magazin rather than an ammo belt is a nice touch. I also like Hellboy's revolver. It makes the Smith & Wesson 500 look like some cheap .22 revolver. I recently got Halo 3 for the X-box 360 (My parents got it for me and my brothers on Luminary Sunday) and I love the assualt rifle the humans use. It's basically a FN F2000 with coverings, chambered in 7.62x39mm, and with an ammo counter added on top.