I voted for Vex because they have time travel and a logistical edge. But the Hive are definitely up there. Vex are on top because they didn't just take over Mercury, they Borg converted it. And most other planets, except apparently Earth, have ancient or semi-ancient Vex structures hiding out. The Cabal are all about digging in like a tick, but they wish they were as embeded as the Vex. The Cabal clearly have some scary power, such as supposedly blowing up planets, and knocking moons out of orbit. But I think they're the epitome of slow and steady. Since we don't know whether or not they were part of the Collapse, but most likely not, they seem to be the lesser threat of all at this point. They showed up to fight with the Fallen at the Twilight Gap, and since then haven't budged past Mars. The Fallen are spread our and kind of a wild card, but they're mostly rebellious criminal pirates. Hell, an Awoken took over one of (or most of one of) their houses. We've even fought alongside them. I almost feel bad killing them. Except little b1tch Captains. F* Captains. Stop hiding. The Hive are a huge threat. They've essentially terraformed the Moon. They're right on our doorstep. They tried to summon their god from another dimension to destroy Earth. They wiped out a huge chunk of Guardians during the attempt to retake the Moon. They may have actually wiped out more Guardians (in lore, not in game players) than the other factions. But the Vex are the worst. Control over time. And almost control over reality. We could all be a simulation in Dr. Shaw's office, thousands of years ago in real-time.