If you're at a close range to your opponent, use the Assault Rifle, this is good especially because in most playlists it's already listed as your secondary, if the DMR is permitted. Sniper rifle is a very suprimitive weapon, and is a great support weapon if you have trouble with hitting a head shot, and get a clean up.
Grenade launcher has a very large blast radius and when combined with the DMR effectively, its' EMP properties can play in your favor.
Good luck
"Where is my supah suit?"