Posting this here because why not.
Athrun/Yukiko:
This. Enough said.
Athrun/Seolla: She's a tsundere, he dated a tsundere. And Athrun's fighting style complements hers.
Shirou/Rin: My Type-Moon OTP, and yours especially is amazing to thread off of and ship with. I have so many good memories of Anemoi threads and logs involving these too.
Shirou/Caren: He's pretty much the only one who cares enough to try to work his way past her cold exterior and...weird behavior. And when he does, they have cute moments.
Shirou/Kyoko: We haven't done a whole lot with these two, but I like what we've done. She's entertainingly active, which is a lot of fun to play Shirou off of.
Asem/Seolla: Belligerent. Sexual. Tension. Holy
shit.
Asem/C.C.: I'm still amused by the parallels it would create with Asem's father. Plus, with Asem's temperament, I think he could get her to warm up to him.
Ike/Eirika: I like how they're both just simple, honest characters. That's what makes them enjoyable.
Roy/Cornelia: Feeling this one the most lately. They had some of our most incredible threads, and their banter was some of the most glorious stuff I've ever read or written.
Ky/Kaguya: They're just too sweet and/or amusing together.