Hm, I actually have to think quite a bit about this, interesting. I’m not sure! I quite like…
Rarijack, because they must have a ton of sexual energy together. Basically make-up-sex turned to eleven.
Appledash because of various obvious reasons
Scratchtavia because they’re both adorable and I picture them as very sensual and sexual partners
Twilestia, because I somewhat like the idea of the whole student/teacher concept, especially if the teacher is a goddess. Also they doing some role-reversal in bed would be adorable and hot as hell.
Rarity x Everyone, basically. She just makes such an awesome Dom.
Chrome x IE, because.
Oh and I absolutely love the (albeit creepy) HumanPinkie/HumanDash/Futashy-triangle.