Dungeon Crawl: Stone Soup: Took me 4 years of playing almost every day to beat the game with a Human Wanderer. If you can beat my record, I will be VERY impressed.
Dwarf Fortress: Not for the stupid or faint of heart. DF is a game developed by a unemployed doctor of mathmatics and is just as complex as you would expect. The graphics are all ASCII and the logical algorithms and AI of the game WILL bring your CPU to its knees.
X-COM: TFTD: This is the sequel to X-COM: Enemy Unknown, and for the most part takes place underwater. It's fucking excellent and even easy difficulties will make you want to cry.
Battle for Wesnoth: It's a strategy game kinda like a mix of Breath of Fire and Heroes of Might and Magic - but it trancends both series and has a very unique 'feel'.
There are more, but very few people share my taste in vidja. If anyone plays these and enjoys them, I have lots of other 'gold tier' games in the back of my mind. (of which M&B is definately one)