NetHack

The Good
It is comprehensive: it has great scads of races, of items, of dungeons (quest levels, a Sokoban game, the Gnomish Mines &c.); even the kitchen sink. It never gets old: just when one thought one knew it all, some new thing leaps out. After NetHack, every other game pales and is boring.

The Bad
It is hard, probably the hardest game I've ever played. It'll keep drawing you in and killing you off. Just as NetHack has more items, monsters and types of levels than other games, so too it has more ways to die: being killed outright; starving; choking to death; eating too much food; poisoning; petrification; being crushed by boulders; falling down stairs; donning an amulet of strangulation; and so on and so forth. It is a tough game.

The Bottom Line
Well worth playing. It's available for nearly every platform, and there's even a GUI (Falcon's Eye, I believe) available. It requires thought and consideration--there's no such thing as an unfair death (or any other negative occurrence) in NetHack: it's always winnable, and always possible to think one's way out.