I got all different mains for different missions.

I main:

Volt for sabotage/hive, capture, rescue and spy.

To get these missions done quicker.

Saryn for defense

Quick kills and nukes.

Mesa for certain assassinations and high index.

Fast(er) kills

Trinity for hijack, sortie assassinations.

Keep shield up, and ev takes a huge portion of health of some bosses that can be EVed.

Oberon for survival, defection and infested salvage.

Keep health up for kavor, team alive, and tank.

Banshee for mobile defense, interception.

Sound quake holds most enemies back, and makes these missions 10x easier.

Chroma for tridolon hunt, certain sortie missions.

High damage, armour.