I think the Hive is the most terrifying in the classical horror sense. I mean Bungie really did a good job with all their screaming! However, IRL, I think the Vex would be more terrifying to face off against, because of the teleport, and relentlessness. I think I can at least understand all of the others on some level. If you run at a Thrall, he'll back off. Witches will hide, but I've never seen a Vex do that. Well, maybe the flying triangles.