About to get my 1st zeta who should I put it on?

Based on your collection at the moment, Vader isn't a bad choice. If getting gear on other heroes isn't too long of a push, Kylo isn't a bad idea either. I will say a 7* Barriss with zeta helps me break a lot of things in half, but i know she's not fully promoted yet. Hard to tell, seems there are new ones coming out all the time now. You could save, but that's so hard and unpopular.