If we're talking any conventional firearm, it'd be an HK MP5SD hands down. I've always wanted one of those. With the 0-1-3 trigger group. Burst fire plus no need for subsonic ammo, mmmm. It'd be a really fun gun to have
But because there is no way a civilian would EVER be able to own one without a mountain of paperwork and money to b e legal, I'd have to go for a PSG-1 with the specialized scope.
For a totally "no frikken way anyone outside of the military could ever get one" purchase, it'd definatly be an HK21E. The description on HK's webpage states:
"...HK Machine guns deliver semi-automatic sniper-rifle like accuracy from a belt fed machine gun..."
Happiness is a belt fed weapon
Edit:
Woah