Shaak ti unique is the first one because it makes the team stronger. Ct5 savior, after that ct5 turn meter gain is very important to get rex is flying kill attack as fast as possible.
Rex zeta do not affect himself. So this zeta is good for echo EMP and ct5 assist attack only (until we get another clone). It’s good but not as others.
Rex unique and shaak ti leader are the two last. You need 5 zeta to mostly complete them. Echo zeta is very random because it triggers on evade. So it is mostly designed for tbls i think and it is not very useful in GAC or tw.