I'll have to go with the republic, simply because it's about the only side that really looks different from the original game. I really like the designs for heavy trooper and sniper, in particular. It's also nice to see that their "command skirt" thing actually moves around when you walk.
Second would be a tie between CIS and Rebels. CIS because I've always liked the droids (since I first saw them in TPM) and rebels because their "commander" is very unique.
As a side note, does anyone know why the music that plays when you play as Grievous is actually the flag music from the TPM podrace? That's pure nonsense.
~Dark Cupcake of the Sith