In my opinion the Confederacy's is the best.
Feel free to share your opinions below.
(And yes I excluded the Old Republic and Sith Empire.)
Edit 1: Have you guys have even listened the Confederacy theme? (This goes to the people who voted empire.)
Edit 2: I just realized that I spelled alliance wrong, autocorrection is a bitch.
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Thw confederacy theme is amazing. If theres one thing done right in the prequels, its the music