Well, I would probably pick up almost any weapon I could find, but if I had a choice it would be either:
Automag/SMG
Sniper/Automag
Rocket Launcher/SMG
I will probably never pick up the plasma pistol. I haven't got I kill in multiplayer, and as far as I can remmember, I never picked one up i campaign.
And to everyone. There will be no dual-wield.
p.s. Does anyone know what the ammo cap for the SMG is? Is it as the original Halo 3 smg or have they lowered it to compensate for the increased accuracy, range and added scope?