Well really its whatever you want to use....what gun can you do good with. Its the Perks you need that helps. If you can rack up kills use Hardline, quicker kill streak rewards. Slight of Hand....faster reload to kill alot more without dieing while reloading. And Marathon, so all you do is run run run
If you run out of ammo, Pick up some dead guys gun....always helps
Oh btw dont run around corners and try to use grenades or semtex.