WrongSprite said:
I'm gonna say Dwarf Fortress. Man, I wish I had the patience to learn that game, but the learning curve is such a sheer cliff, that I just can't be fucked. I can imagine it'd all come together at a certain point though.
I'll second Dorf Fortress here, since it's pretty much the definition of "hard to learn, easy to master".
Dwarf Fortress is almost impossibly hard to get into, having amazingly deep and complex gameplay tied to an amazingly bad interface. The fact that its graphics are extremely confusing for new players doesn't help at all, nor does portraying a 3d game in 2d layers.
Once you get the hang of it though, it practically plays itself. Seriously, once you get good at Dwarf Fortress it becomes almost impossible to fail unless you do something really stupid or mess things up on purpose.
Dwarf Fortress is most fun in between those two stages: The part where you're good enough to understand what's going on, but still new enough that new situations or unexpected developments catch you by surprise every now and then.