I highly recommend Dwarf Fortress. The Steam release makes it so accessible, but what' cool is that it still astonishingly deep and complex. I don't mean that you'd be surprised how complicated and obtuse some aspects of Dwarf Fortress can be. It's more that by Steamifying this game with great graphics and menus, you realize that its lore goes far beyond its original-- crushingly hard-to-master--ASCII interface.
What's most compelling is how it creates rich and surprising narratives and tricks your imagination into doing some of the heavy lifting for the game to be so addictive. Such great storytelling -- some of the best stories I've ever encountered in gaming. And it's procedurally generated. It's really a work of art. Hilarious, sad, poignant, endearing, and sometimes, very, very bloody.
Ed Bott