Ranked without Zetas:
Hera - Just needed. No choice, here.
Kanan - The tank. Gives the counters to everyone so, again, no choice.
Zeb - Survivability on passive is insane and his array of debuffs is vital.
Ezra - The best damage-dealer, plus some good utility.
Chopper - Dispel is useful, tanky passive is helpful, not much use outside of that. A 1-turn taunt doesn't do much good unless you're filling the gap between Kanan taunts.
Sabine - Does good damage in relatively uncommon circumstances, AOE is good, but not amazing, armour shred will rarely stack high enough to be useful.
Ranked with Zetas:
Hera - Still no choice. Zeta is also pretty damn good, so a nice bump to an already good character.
Kanan - Zeta just moves him up from 'the only decent tank in Phoenix' to 'a fantastic tank, especially in Phoenix'.
Sabine - Almost as much damage as Ezra, even more debuffs than Zeb. Suddenly, very good.
Zeb - Still great for debuffs, but now his stick is counted as a WMD.
Ezra - Still the best for damage, but that's pretty much all he does. Zeta doesn't try to change that.
Chopper - Has no Zeta. Too bad, fella.
Zetas ranked:
Kanan - Adds a team-cleanse, gives them more survivability, and seriously nullifies his speed issues. One of the best Zetas around.
Sabine - Staggers and Exposes for everyone! Plus, gives Phoenix allies buffs, reduces cooldowns, and adds more damage. Fixes many of her issues and makes her almost irreplaceable.
Hera - More potency, cleanses, gives TM, and reduces cooldowns. If Hera's fast enough you can have Kanan perma-taunt with this Zeta. Which is, you know, brilliant.
Zeb - As I mentioned, his stick becomes a weapon of mass destruction. Reduces the CD, can't be evaded, gains him TM for Phoenix allies, (if the target has no debuffs) deals 75% more damage plus 20% health damage, (if the enemy has buffs) ignore armour, (if the target has debuffs) deals 50% more crit damage.
To sum it up, if you tap on an enemy and then tap on this ability, you will ruin their entire existence and, possible, tear a hole in the fabric of reality.
Ezra - plus 50% damage, 25% more TM gain, defence up. EHhhhh. It's.... okay. Makes him hurt more, but.... ehhh...