Good question.
On large maps I use M4 Socom , M16/M203 , or sniper with the good ol' M24.
On small maps its AK47 , M16/M203 , MP5 ( Purely for the joy of seeing people dance )
But then another thing comes to mind when I think of weapon selection, If Im a Ghost I use military issue weapons, and when Im a terrorist/rebel its AK47 all the way. ( Its rare for rebels to have high tech weapons unless they have weapon smugglers or stole military shipments.
It makes the game more realistic if the weapons match the person using them, of course this aint cops 'n' robbers. lol