Say what you will about my choice, I realize it doesn't have big ass explosions or anything like that but I love it! It's the M1 Garand, particularly from Battlefield 1942.
<spoiler=Image of M1 Garand>
For a number of reasons, I loved it's semi-automatic fire it fired as quick as you could press the button and did it without even a single reload animation. Also the recoil never seemed to bad to me.
Also it was just the sound of that *PING!* as the cartridge ejected itself out of the rifle. I remember the first time seeing and hearing it; it just seemed so awesome. I then understood why it was so detrimental in World War 2.