This doesn't mean sexual couples because (disdainful) I don't Do That any more. It means binary relationships. And nobody has to try and find a pattern here, because I doubt there is one.
In no particular order:
1. Gou Jian/ Ya Yu (Woxin Changdan)
-- Gou Jian and everybody, in fact, because he's *so* much all things to all men-- or different things to each individual. Gou Jian/ Ku Cheng, Gou Jian/ Wen Zhong, Gou Jian/ Fan Li, Gou Jian/ Ye Yong, Gou Jian/ Wu Zi Xu. But Ya Yu is the most satisfactory and unpin-down-able relationship.
2. Goujou /Hakkai (Saiyuuki)
-- returning to my roots here, the satisfaction of that tacit friendship whose unspoken boundaries take in most of the known world
3. Haruka/ Michiru (Sailor Moon)
-- still the acme of how f/f should be done
4. Elizabeth/ Darcy (Pride and Prejudice)
-- just because one would rather like to be married to either of them
5. Fan Li/ Wen Zhong (Woxin Changdan)
-- old marrieds. I just wish I knew what they were *saying* whn they snipe at each other.
6. Aoarashi/ Ritsu (100 Demons)
-- people who get on each other's nerves but can't do without each other
7. Koh/ Kubota (Wild Adapter)
-- always wondered what that's all about. But neither is the kind to talk and tell you
8. Marlowe/ Shakespeare (RPS)
-- if someone would just do it *right*
9. Hakkai/ Hakuryuu (Saiyuuki)
-- Hakuryuu's devotion is the purest in the series. Even Gokuu's devotion to Sanzou has a reason, but Hakuryuu's just is.
10. (shameless self-indulgence) Goukou/ Hisui
-- I have a thing for managing types, especially when they manage with grace
What we need is a meme for the Solitaries, people one wants to see in a relationship with someone but for whatever reason doesn't, short of fanfic. My own shortlist:
1. Vetinari (Discowrld)
2. Keiki (10 Kingdoms)
3. Katse (Ai no Kusabi)
4. Kinu (100 Demons)
5. Goujun (Gaiden)
6. Ageha (Basara)