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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Cabal are my favorite. They may be big bit they are not mindless. It seems like they devote all their intellect to war and conquest. Mindless races can't make weapons to blow up moons in thier way :-)