A plasma weapon destroys shields faster and a human weapon overloads the "damage threshold" for unshielded players, thus the best combo is always an Alien-Human combo.
1.) SMG and Plasma Rifle
2.) SMG and Plasma Pistol
3.) Magnum and Plasma Pistol
But don't think that these are better than just human or just plasma. Human-Human and Plasma-Plasma weapons work good too.
1.) SMG and Magnum
2.) Dual Magnums
There is also a very wide variety of good choices between your primary and secondary weapons but that list could go on for pages so go play and learn them.