A lot of my favorite pairings have unresolved sexual tension. Maybe even most of them. It's a turn-on of mine. So it's hard picking out a specific pairing that I haven't already used.
But with Yoruichi and Soi Fong, the sexual tension isn't just a slight edge that pushes the pairing along in my brain; it's practically the engine driving them. The thing is, the unresolved emotional issues are not just sexual. There's a hundred years of built-up hurt on Soi Fong's side, and a near-forgotten tender protectiveness on Yoruichi's. But the sexual element lurks under it all. Yoruichi uses potentially sexual language to greet Soi Fong. Their fight is personal, involving hands and touches more than big dramatic swords. And it includes progressively less clothing as it goes on.
So you get this combination of sexually charged subtext and intense, unexpected emotions. All of which leads up to this: goddamn, they had better have had sex after that fight, or there is no justice in the world of Bleach.
Oh, wait. There is no justice in the world of Bleach.