I've been developing Phoenix as a side project. They are actually quite useful in GW, where they can take tons of nodes and end at full protection and health. Here's my initial thoughts:
Hera is the glue. If you are building Phoenix as a squad, you obviously want her to survive.
Zeb is the MOST important toon after Hera. Inflicting Daze is key. You have two other toons that can call assist to turn that Daze into a stun.
Kanan's individual abilities kind of stink, but his unique is a MUST HAVE for Phoenix. 70% counter? Yes, please, thank you very much. 25% health when suffering a debuff? Sign me up. Kanan is what makes the whole team really lay down the damage. Plus, whenever enemies AOE, Zeb sweeps in and adds his Daze 70% of the time.
Ezra, so far, is better than Sabine to me. The ability to call Zeb for the daze-to-stun trick is nice. And, oh, btw, that often turns into three attacks on a target. Ezra also dispels and goes lightning fast.
After that, Chopper or Sabine go in depending on the need. If I need to dispel lots of taunts, and the enemy isn't applying a lot of debuffs, I choose Chopper. Otherwise, Sabine.
Pick your development strategy based on these notes, but bear in mind that they are based on working as a team. If you were going after a single toon, I would say Ezra. Calling a TARGETED assist is wonderful.