I have 14 Zetas and confirm that Finn is a great choice. Resistance is amazing in HAAT. They work well in both P2 and P4. They routinely clear my entire GW, beating even the most well modded meta squads (thanks to the TM head start). They aren't meta right now in the Arena, but they are viable, and with the new movie coming out you can bet they will get some love.
Thrawn and R2 are both great as well. However, given where you are in the game, Resistance will make the biggest difference IMO. True Thrawn can solo P3, but it will take a solid hour, and you will need very strong mods to do so. He will also require both zeta's if you are planning on running him as a leader. R2 is another good choice as he benefits so many different factions, but again you won't see the return like you would from Resistance.