Personally, I am most intrigued by the Fallen; it's just interesting to consider the dynamics of each House and how they fight for power over one another yet work together to fight the Traveler (as evidenced in the Battle of Twilight Gap). That, and the concept of a fallen race (ba dum tsh) fighting for redemption to their former glory, makes for an interesting enemy to face.
On the other hand, I despise everything about the Cabal with a fiery passion, and I wish for the Psion Flayers to lose control of Phobos so that it careens out of control and crushes the entirety of those mindless brutes.
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The hive, all of them are twitchy and move alot, their scenery is boring(the moon) and caves are so one way and tight. They scream like rabid banshee's. Wizards are assholes and phogoth is a giant bullet sponge.