More accurately, My Little Axiom of Choice states that there exists a function mapping the set of all bronies to the set of all partially ordered sets of ponies containing a maximal pony (poneset). In short: every brony has a preference.

The Worst Pony Conjecture states that there exists a pony Twist such that if Twist is a maximal pony in a poneset then said poneset is not in the image of the MLAC function. The existence of Twist is a hotly debated topic and the mention of her name often causes conferences to become riots.

(Thanks to emnt for the Axiom of Choice suggestion!)