Oh that's a tough one. Does Loki have his magical stuff or Odin's spear? Same goes with Sephiroth. Does he have his masamune sword?
Sephiroth: He can fly, has uber fast reflexes, master swordsman and he can slice buildings with one attack. Also, Sephiroth has never display the full extent of his power due to cat-playing-with-the-mouse behaviour. Forget that he can projectile his attacks too.
Loki: magic spell manipulation - mind control, master of illusions, concealing himself or others (Thor's deleted scene), manipulation of magical relics: odin's and thano's stuff, casket of ancient winters. Extreme stamina, superhuman reflexes, and proper fighting skills above human level - His battles with Thor show him equal in fighting skills and sometimes he may caught off guard the son of odin (although the help from the magical stuff does help).
Thor: If Thor can manage to defeat Loki twice (in the films) due to his more brute strength and the use of Mjolnir, then it will be between Thor and Sephiroth.
I can be biased for Sephiroth since we do not know the extent of his power, thus he may overtake the son of odin. Interesting fight!