Androids are in contention for getting the most. If nothing else, they get a lot of personnel with the species, making it easier to have teams which can complete missions in play while still holding on to three Androids in hand. I would say they got the most straight-forward "boost."
AU TNG got a small handful of personnel. They're pretty good, especially Ogawa, but probably not as dramatic a boost as the other themes in the expansion.
I'm not sure I'd call the Bajoran Prophets theme a "boost" so much as a new theme. At the same time, Sisko might be the single most impactful card in the expansion, and you don't need a lot of support in your deck to use him (just a handful of Prophet cards).
I think it remains to be seen whether it's worth running Storm the Castle or if Cardassian Dissidents will be better off with a more flexible gameplan. The new slate of non-unique personnel help a lot, but they work just as well at "normal" mission as they do on the flipped HQ. Still, the Storm the Castle plan seems like a ton
of fun, and I'll play if for that reason even if it turns out not competitive.
In the end I voted for Khan. I have to consider not just the new cards, but the pending change to Ceti Alpha V. I never enjoyed playing them without an HQ since Joaquin was such a single point of failure. Patching that point of failure makes me want to try a Ceti Alpha V-Regula-Genesis Planet deck. And fixing To Rule in Hell (which, my understanding is that it was always meant to be 100) while simultaneously giving Khan ways to spend those extra points makes me want to build a 4-planet deck. Running them from an HQ might still be the "right" play in terms of competitiveness, but at least I will try out Ceti Alpha V now. I would say going from "would never consider playing" to "am excited about trying two decktypes" is a huge
boost, and that's why they get my vote.
Honorable mention: AQ Borg, specifically with Population 9 Million - All Borg
and Jouret IV
check a lot of boxes for that decktype. Saturn directly helps Pop 9M, while Jouret IV qualifies for the Queen/Locutus protection. Combine with Earth
for 100 points, letting you spend what you get from Annexation Drone
(or have it protect against point loss). Plus now we have enough
missions to let Implacable Scourge
cheapen your Maneuvers. Oh, and the icing on top? Rishon Uxbridge
protects Pop 9M.
"Save your strength. There'll be another time. The princess - you have to take care of her. You hear me?" -Han Solo to Chewbacca, ESB