It's a toss up between OR, Sith, Mandos, and Vong. It's like they planned it that way!

Seriously though, I'd say OR got the biggest boost, but that's partly because tehy had the farthest to go. Revanchist and Bastila are both awesome pieces which help them out a LOT. Sith or Mandos come in second; Revan, Exar, and Bandon might have just brought Sith to the top tier, but Jaster helps take care of the Mando auto-loss against Lancer. If you were to include Jaina with the Mando options, they would definitely get second with Sith at third.

Vong got some of the best boosts in the set, but it wasn't from the uniques. Yammosk is a total game changer for them. Nen Yim is a great addition, but without the Yammosk she would not have been a huge boost. Shedao is neat, but he's really just a Jedi Hunter with extra tricks (not that he is a bad piece by any means, just not a game-changer). Shimmrra is another good one, but without the Yammosk even he wouldn't have done it for the Vong. So based solely on boosts from uniques, Vong would be #4 (although overall I think they got the biggest boost out of any faction due to the Yammosk).