My favorite level concepts are the alien concept, & the extremely abstract surrealistic concept. Both of these concepts are my favorite since I just really like them. The alien concept is well, just alien, with many things that are unknown to humans, & very strange but interesting to look at. While the extremely abstract surrealistic concept adds some corrupted but also extremely strange environment, breaking all bounds of physics, & the universe.
So yeah, what's your favorite concept for a level, answer & discuss :)