I'd go with either Daka (for stun/heal/rez) or one of the major damage dealers, GS/IG-86. Alternatively if you have Ewok Elder, I'd prolly use him, since he can heal/rez and most importantly, clear status effects (to allow Rey to operate at max efficiency)
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More damage is almost never a bad option.
Daka is a great stunner, especially if you omega her basic.
STH has become fantastic, and his taunt makes him useful in any lineup.