Slash Dementia said:
The Cradle in Thief: Deadly Shadows... That level was fun.
This. Though I don't recall initially found it "fun" as such, but then again, I that's probably because I was scared witless by the place

.
Also:
Surface Tension & Questionable Ethics from the original Half-Life.
Intestinal Fortitude in GoW2 as well.
Ice Cap Zone from Sonic 3 - yeh, it's an ice level, but had one of my favourite BGMs in the Sonic series.
Not strictly speaking a level, but the Netherstorm area from WoW. Although the place was mostly desolated, I liked the fact it had a couple of biospheres dotted around that were full of greenery and wildlife.
Hyperblast & Morpheous from Unreal Tournament.