Originally, I thought that this is a tough one because I would equate challenging with complicated. I tend not to like complicated, so-called “crunchy” systems generally. However, then I remembered my younger gaming days and it came to me.
It’s a very crunchy game system, full of numbers and statistics, but once you get past that, it’s actually a very good game system. We played GURPS (Generic Universal Role-Playing System) for ages and had a great time with it no matter what setting.
As you got to know the system though, there were ways of bending it and twisting it to exploit the various crunchy bits that made it what it was. At first, that was fun, as you were able to make pretty powerful characters fairly easily. But, it grew tired and eventually I moved on.
I remember GURPS fondly, but I’m not sure I’d go back. The idea of a universal role-playing system is quite common now (Savage Worlds, Apocalypse World Engine, D20, etc.), but I’m no longer as interested in a challenging system. I’d rather one that gets out of the way and lets us play, but lurks in the background ensuring our play is still a game and not just round robin story-telling.