I have Sabine and Kanan zeta'd. Ezra was my strongest of the Phoenix squad prior to that (I have him at g12) and now he's third and I'm debating a zeta on him.
I chose those two zetas first because they provide better team benefits than the others. Both are awesome, highly recommended (Sabines gives big offensive advantages and Kanan gives big defensive buffs), but I don't think any of them are great for synergies outside of Phoenix.
Not sure if that helps but if you are considering a Phoenix zeta I would look at those two first because they provide better value for the whole team.