Water Level

Usually the hardest and most hated level in any video game. This is because the physics and spatial reasoning of being underwater are usually very different from most other levels in a game, meaning that you can't rely on the muscle memory you've built up in previous levels to carry you through. Also, they tend to have very hard (and very scary) monsters.

Tim: Have you beaten Zelda yet?

Jim: Naw, man, I'm stuck on the water level.