My vote is for Rakushun! LOL. Isn't that obvious?
I agree with all your points about Keiki. Also, I think he's a bit too serious for Youko . . . Can anyone actually see them having fun together??? It's probably also against Tentei's law.
While you have a good argument for En-Ou, I think you're forgetting one important thing. Even if En-Ou does understand Youko's position better than anyone else, the very fact that he *is* an emperor would cause difficulties. I mean, it would be one heck of a long-term relationship . . . and I highly doubt Tentei would really approve of two rulers getting together. Wouldn't that totally mess up the heavenly plan of keeping the kingdoms independent? I mean, even if the two of them managed to keep their personal and professional lives separate (think about how hard that would be) other kingdoms might feel threatened by such a situation (because the rumours, at least, would get out). En-Ou also doesn't seem like the type to settle down. It HAS been 500 years, after all . . .
On the other hand, I think Rakushun is in the perfect position to be both advisor and romantic partner for Youko (well, once he's done repaying his debt in En and assuming someone enrolls him in those lists of immortality so he's not 83 years old at that point) since he can play both roles with no conflict of interest. It's true that he would be in a subservient position to her (I mean, she IS an empress) but Shoukei's mother was the royal consort (notice they didn't call her empress) and it's not like that mattered. I also think Rakushun would be good at Youko's side because he sort of plays the role of everyone's conscience and he knows how the people think and live and understands them from the point of view of a person who was born and raised in that world. He can help guide Youko and keep her on the right path better than anyone else. I mean, she has respect for En-Ou and Keiki but I don't ever think she'd dismiss Rakushun's advice out of hand. (Unlike *cough*Keiki*cough* - tho' she's getting better there.) And despite being a half-beast, he's such a great guy I think the people would all grow to love him.

(Which saves Youko a lot of headaches down the road - once again bringing Shoukei's mother to mind)
Okay, but my real reasons?
He's cute; he's sweet; he's wise; he rocks! And I think he's rather sweet on Youko (even if he won't admit it) and I can really see Youko returning the feelings after a while (she did give him that bird, after all, and he IS one of her very best - if not best - friends). Plus, he understands her better than anyone else. (Keiki still gets confused.)
BTW, I'm assuming an Emperor/Empress can get "married" (in the "living together but not having children way") after ascending the throne because Enki asked En-Ou whether he had finally found a wife upon seeing Youko that time . . . or was he just joking around?
I do think Youko and En-Ou will continue to be (and probably become even moreso) good, close friends and companions. Because En-Ou is really cool too.
BTW, I find it sort of disturbing that I've found Japanese sites pairing Youko and Enki up . . . o_O