If those three are your only choices, then zeta Sid, as for a best zeta, there is no one magic zeta, it's whatever chars you have and whatever team you have that you like running
As far as Omega's go for those three omega EP first, then Sid, and don't bother with wasting an omega on Imperial Phalanx unless you only use RG for the AAT raid.
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Very sound advice. Whom do you think has the best Zeta ability?