Way back in '95, my college roommate introduced me to Doom and Doom II. His favorite level in Doom II was Barrels of Fun, because it was impossible to beat it without cheating (he liked the idea of the programmers giving a hearty “fuck you” to serious gamers… yeah, he was sort of a jerk): You start the level completely surrounded by explosive barrels, such that there was nowhere you could go that wouldn’t be within lethal range of some barrel or another. Well, I figured out how you could beat it without cheating: The barrels don’t all blow up at once, so if you hit the ones further away from you, and then immediately run towards the explosions, you’ll get out of range of the ones near you before they go off. Once I realized that, it became my favorite level.
Another one: In Starcraft: Brood War, the level The Battle of Braxis. It’s a tactical mission, where you have a set number of units and if you lose them they’re gone, but after each sub-objective you get progressively more advanced reinforcements. I’ve played through that one many, many times, each time putting different restrictions on myself: Never taking any damage, for instance, or just using my starting units with no reinforcements, or killing everything on the map before meeting any of the real objectives.
Finally, in Diablo II, I think I’m the only player on all of Battlenet who actually likes the Maggot Lair. Sure, it’s a maze of twisty passages all alike, with no room to move past anyone or anything through the narrow tunnels, but it’s also one of the best item drop locations in the game, and absolutely nothing in any of the three levels is immune to fire damage. Hello, sorceress.