Top priority: Hera (Phoenix squad doesn't work without her)
2nd priority: Ezra (he's a legit great toon, even outside the context of PS)
After that, just know you will need Zeb (protection regen) and Kanan (tank / cleanse).
Fifth spot goes to Chopper or Sabine, depending on your taste or preference. If I had to pick only one, I'd take Chopper. Mostly my strategy with PS is to have Ezra go as often as possible (assumes zetas).
* Ezra: Flourish (Ezra gains 75% tm)
* Hera: Play to Strengths, call Ezra (Ezra gains 50% tm)
* Ezra: Watch and Learn, call Chopper (Ezra and Chopper gain 40% tm, 20% chance to reset Ezra's cooldowns)
* Kanan - Total Defense gives Foresight to Ezra, and 50% tm when it expires.
* Chopper - whenever *anyone* attacks, Chopper has 30% chance to attack, and thus another 20% chance to reset Ezra's cooldowns, and hopefully set him up for an early Flourish or Watch and Learn.