On any given day, I'd say Vex. But remember, Oryx had the vex prepared to accept defeat, and he probably would've conquered them had we not stopped him. Who is to say Xivu Arath and Savathun aren't still capable opposing or even conquering the vex. The hive are steeped in a greater darkness then the vex, who I consider to be a young, new entity despite its time travel capabilities. Future vex should be more dangerous then past vex. We haven't seen that, and there is a probably a reason.