I assume you mean humans only, because it would differ depending on which species you are too.
For Soldier, max Concussive Shot and spam it. For Adept, you of course won't need a weapon, so choose whatever gives you +200%. And use biotic detonations constantly. Engineer, give Combat Drone the shock ability over the rockets for the last upgrade. Otherwise I've noticed that it just stays in one place and doesn't seek out new enemies to pester.
Weapons don't really matter. But you should never have more than one.
--
~Halo Live, fight, survive, as a family.
http://img267.imageshack.us/img267/7469/batfamily.jpg