It's Ackbar. I'm on a July 16 shard and have:
Ackbar / g12
Thrawn / g12 / double zeta
Tarkin / g11 / double zeta
And the three geo pilots at g11, 7*.
I've experimented with them all and it's overridingly Ackbar. I hit top 5 at payout, and fall to top 10 overnight.
Main reason I use Ackbar is because you can use his aoe very quickly which will call at least 2 assists, and will leave one enemy in at least yellow health.
Biggest difference I found though is putting an omega on spy so he and soldier stealth when spy takes damage. Then first re-enforcement is Scimitar to keep it up and make Sun Fac tank regardless of taunt (so second omega should be on him for the extra defense), then plo koon to restore 40% health & prot , & dispell all debuffs. Vader & boba are the others but rarely need to call them.
Final tip, take out a FTP early on using any attack that calls assists.