well that depends on what type of games you will be playing (open field/CQB)
what type of weapon design you prefer (M4/M16/AK47/G36)
on this site you'll find plenty of reviews of a lot of weapons.
personally I would suggest a M4, its short so you can use it for CQB but long enough for open fields,
there are a tonne of after market part available from rails to stock, to sights, to magazines.
its a greatly versatile weapon
"You know what the chain of command is? It's the chain I go get and beat you with 'til ya understand who's in ruttin' command here."