All things equal that's a tough choice. It really depends on what else you want to add to the team synergy you already have going with the current team.
If you want a second counter attack member on your team then go with Kit Fisto.
If you think you're currently missing the utility of a buff remover then go with Mace or Plo.
On one hand Plo can dispel a positive status effect with every basic attack he does, and Mace can only do it with his special attack (not sure what the cooldown is) but then he also provides all allies with extra attack meter if he dies.
Who's the leader in your current team?
Edit: I'd also like to add that I'm very jealous of the Jedi characters you have to choose from right now
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If you want a second counter attack member on your team then go with Kit Fisto.
If you think you're currently missing the utility of a buff remover then go with Mace or Plo.
On one hand Plo can dispel a positive status effect with every basic attack he does, and Mace can only do it with his special attack (not sure what the cooldown is) but then he also provides all allies with extra attack meter if he dies.
Who's the leader in your current team?
Edit: I'd also like to add that I'm very jealous of the Jedi characters you have to choose from right now