^Hah, I finished Tsukihime like a year later.
I don't see Arc and Ciel ever working out. They are the perfect frienemies and while Arc would definitely mess around to tease her, Ciel would never ever go along with it.
Shiki and Ciel also didnt do much for me, but Shiki & Arc is quintessential Type-Moon. They are the poster couple next to Ryougi/Mikiya and Shirou/Saber (as much as I hate the fact that it's not Rin). As far as a poly ending goes, only Shirou/Saber/Rin ever convinced me. Maybe Hakunon/Nero/Tamamo too.