In no particular order.
RG - Auto taunt when you need, stun / speed down on basic and a massive health pool. He's amazing.
Rey - Broken multi-attacker with built-in Evasion and crazy fast. She can kill a squishy all by herself and requires at least two chars to kill her.
GS - Crazy speed, crazy damage, crazy high health. All he's missing is perma-stealth.
Daka - Best healer in the game. Stun (double-stun!) on basic. Auto-res, heal + res, good health, good speed.
QGJ - Crazy fast, makes Jedi crazy fast. Hits like a truck, eats taunts and makes the team hit harder. Relatively low HP to make up for it.
Dooku - Crazy fast, stun (double-stun!) special, 100% counter-attack, double-attack basic, arguably game-breaking Leader ability, even stronger vs Jedi teams (meta)
Phasma - Great HP, great turn manipulation, great Leader ability when it works.
ST Han - Definitely second-best tank, hands down. Taunt forever, making the rest of your team take lots of free turns while the opponent tries to wear you down.
Poggle - Insane speed, Offense Up team buff, decent HP, Ability Block basic, especially bad with GS / droids.
Fives - Great HP, great counter chance, double-attack basic, decent damage. Really hard to take down and hurts your opponent while they do it.
There are some others that are close to the list, certainly viable... Just not the best. These are all characters that I take note of (especially in Lead for some of them) when I start a battle.