Most challenging but rewarding system have you learned?
Hmm, I've learned lots of systems. Although, unless you have a math or science degree, you never really learn Champions (at least the 1980s edition). That was challenging but meh on the rewarding. Ironclaw was rewarding, but not particularly challenging to learn.
I think the answer is Fate. It's not really a hard system but it's a very different mindset than the games I've been playing for the last..few decades. That made it a challenge for me to learn.