Sion, DN/Thrawn, then Vader
all uniques
that's my two cents. Sion's makes him a lot better and makes it easier to get to Held By Hatred. Vader's already plenty tough enough, and you've got plenty of debuffs around so you don't really need DN, but you've also only got two other Empire chars to proc Thrawn's unique, which means it's pretty much for tenacity and counter chance, which isn't really zeta worthy. If Traya looks to be within reach for your guild I'd save a zeta for her.
tbh I'd get Sion then hold to see if a zeta in another area would be more significant unless you really think those small boosts would be significant.