“Most challenging but rewarding system you have learned?”
Burning Wheel, natch.
IMO, there is no RPG as tightly designed as BW, no RPG that demands as much of its players, and no RPG that better rewards that hard-won mastery.
BW is hard. Learning BW is like learning to play an instrument; there is a lot of woodshedding and trial-by-fire involved before you’re making beautiful music. But once you’ve got some sills, and you sit down to jam with some other skilled players, it’s like nothing else.