For me, it's all about several factors.
-What I'm going to be fighting.
-Where I am fighting.
-The abundancy of that weapon's ammo.
Most of the time, I am using the Assault Rifle, DMR, or Shotgun. In Campaign, the same thing. With a few exceptions:
Exodus: Assault Rifle + DMR
The Package: DMR + Shotgun (and then a Grenade Launcher later)
Lone Wolf: Sniper Rifle + Shotgun (and then an Assault Rifle when I run out.)
I never really use the Magnum unless I have no other choice. The same with heavy weapons and Energy Swords. I have a rule of thumb: "If there's not a replacement, don't grab it." And it's worked great for my Legendary runs.