I say Vex, however HoW showed that they might one day be at a disadvantage to their own technology. If it wasn't for the Guardian and Reef interference with Skolas then there is a good chance that the Fallen would have been able to aquire the same type of tech that the Oracles use. Which would be a problem in Fallen hands. The ability to negate aspects of reality would allow a single Kell to unite all of the Fallen houses into a singular force, which seems to be the leading reason the Fallen are lossing. This along with the ability to transcend time would easily give the Fallen the edge they need to over power the Vex, which are seemingly not tatical at all... Teleport in, walk slowly at enemy, repete.... The Fallen already are seemingly winning the war on the Hive, as most locations on the moons surface are populated by the Fallen with few Hive, and the moon is their home.. they can't even play in the yard. I also feel like the Fallen's numbers and stealth tech gives them an advantage on the Cabal, that seemingly just prefer to blow śhit up.