Speaking of random generation, there's the presiding king: Dwarf Fortress, an incredible game once you get into it, and like its central concept one that keeps doling out treasures the deeper you delve. When raging hordes of zombie mammoths slaughter your hard-won fortress to the last dwarf, you'll know the true value of this unique and infinitely replayable gem. Losing was never so fun...
Yup, if there's one game that's more rewarding if you put a huge amount of time in it it's Dwarf Fortress.
I'll have to go with Morrowind, Dwarf Fortress and something else I'm not sure of.