Most challenging but rewarding system have you played?
AD&D 2nd. Not only was it difficult because it was my first RPG, but as far as all the systems I’ve played to date, it’s still the most complicated. Charts, THAC0, dice, etc…!
Of course it’s one of my favorite systems, so I guess you can say it’s been rewarding.
Most systems I’ve played since AD&D 2nd seem to focus on streamlining rules and having them be less invasive, though Traveller 5 seemed overly complicated. Probably the most complicated I’ve played to date. It wasn’t very rewarding, though. It was a hot mess.