Day 20 - Most challenging but rewarding system you have learned?
Good question. Over my 40+ years of gaming, I've run (and therefore had to learn) various game systems. I would have to say that the most challenging and rewarding game I've learned has to have been the Continuum: Roleplaying in the Yet RPG.
Continuum: Roleplaying in the Yet is a time travel game that is based around an interesting premise of how time travel works (which I'm not going to spoil here, because it's such a central element to the game) with a relatively rigorous scientific approach to the premise, and a set of game mechanics that I really love. That said, its mechanics for time travel are some of the most complicated, mind-bending that you will ever see (aside from having to just deal with the concept of time travel and all that implies in one's head!), and I know that a lot of folks won't even try to play the game. Very challenging on almost every level of the game.
What was so rewarding about Continuum: Roleplaying in the Yet, and remains so for me to this day, is that the game forced me to think outside of the box, and to approach roleplaying from a 4-dimensional perspective, that it made me think of time travel (a subject that I dearly love) in a whole new way, but most importantly of all, it challenged me as a GameMaster, something that I hadn't really experienced in a while at the time of buying and running the game.
I like to think that buying and running the Continuum: Roleplaying in the Yet RPG made me a better GM overall in the long run.