Depends on the character. Can go with in-game recommended mods and just farm away:
Crit damage/Crit chance on Ezra
Speed/potency on Hera
Speed/defence on Chopper
Health/tenacity on Kanan
Crit damage/health on Zeb
Only two I disagree with here are Kanan and Zeb. Kanan I'd throw 4 defence mods and 2 tenacity mods with as many +% protection primaries you can find.
Zeb I'd consider Crit Damage and Potency with a potency cross as well. Need it high to land stuns.
Now that all mods are open to any team it's way easier to mod them.