.45 Long Slide, with laser sighting.
Phased-plasma rifle in the forty watt range
But seriously the SCAR seems to be a nice choice, fits M4 magazines in most cases but nicer touches like a shorter throw selector switch.
A nice metal AK-74 could be fun too.
A M249 will make you popular also.
You could also try to follow the advice of a guy who's not around here too often anymore. "Hit up some games and feel out a few first hand, lots of players show up with a full quiver and while they may not rent them out they'll let you have a look and you'll get the lowdown on bugs if any."
I started with a G36C that I just never wound up gaming with as it didn't click with me.