Personally I’d say Luna, yes starlight is really magical, but Luna has had training from starswirl the bearded (who’s spells are the base for almost every feat made by starlight), also knowledge wise I think Luna knows more spells, also the whole tantapus feat made by Luna seems more impressive and powerful than anything starlight has done, heck her fears in Luna eclipsed itself could give starlight a run for her money, I also believe Luna is more powerful than Celestia easily.