Definitely Phasma, as he can guarantee crits and has an AOE that applies debuffs of his own.
If you have them, B2 and Rex are both solid on this team, especially if you omega B2's special. He attacks and AOE's so often that combined with applying DOTs he can actually do some decent damage. He's also almost essential for beating Rex led teams by baiting out his mass Tenacity Up then using B2 to clear it. Meanwhile using Rex on your own team can provide a much needed speed boost on turn 1 for Vader, 88, and Phasma, all of which have 121 speed maxed, while Rex at 140 speed can essentially boost them to the same 140 speed with his special, as well as clearing any stun or ability block that would prevent them using AOE on the first turn.
Other solid non-AOE options are Leia, who can apply up to 3 DOTs per attack and further increase everyone's crit chance, Rey, who's Flurry of Blows can apply up to 4 DOTs, and RG, who attacks pretty often when not being focused on due to his TM gain, plus stun is very strong combined with DOTs.