Without seeing who you have and don't have, it's hard to give direct advice.
I would say Biggs should be #1, in early game he is plug and play in many teams, great ship and solid in a rebel team when you get there.
Teebo second. Start of a solid rancor solo team. Murder bear with an ewok team in GW, and later game in TW as a good clearing team. Also wicket event.
Resistance Pilot, zFinn is the easiest and best single zeta team out there. So many uses I can't list them all. While RP is not essential she is a good building block to have in place until you have bb8 and r2.
K2, he's good, but not as plug and play, so that drops him down a little on the list. His best lead is Jyn who is not the easiest farm, but he is a decent tank in a rebel team, I would prefer STH at the early game though.