It's between the vex and the hive. If this was vanilla destiny, i would probably go with the vex. Their design and especially their movements, the meausured steps emphasize their robotic nature, and the weird screams show how alien they are, how different. And the "eyes" glowing red are also very good. But with the release of crota's end and the taken king really emphasizes the creepiness and straight up evil of the hive. Most of them are mindless beasts controlled by the ascendant. And that makes them more terrifying, hordes of bug and crustacean like alien creatures with their only function to kill you. But then you see their leaders. Cold and calcuting monsters whose desire is to turn the whole Earth into their breeding grounds, and to smash out any existence of Light. And the design of their leaders is great. Crota in his ascendant realm, eerily glowing and towering over his servants. And then you meet Oryx, beyond massive with his dragon wings and cloak like garb. Seriously, the design of Oryx is amazing. The Daughters as well with their strange pyramid head-esque design, so unsettlingly alien. And the mindless beast Golgoroth, with his mutilated clawlike appearance. So now that we've seen the true terrors of the hive, the big baddies, i can say the hive bosses are my favorite.