Its impossible to choose just one level
I've always loved "A Walk at Night" for sure, to me its the best environmental level. Medic Ted's "Psychedelic Adventure" is incredible, such a pity that he never released a sequel (but check out his unfinished "Brown Lands"). The world in that level just works so perfectly, and is so rich in detail. Oh, and the same goes for Elder's "Uthoha", although its much smaller and very different. Elder always displayed a talent for environments, pity most of his levels are so short and/or based on a single idea.
Then there are puzzles - Dukit's series ("3 Square", "4 Square", "Logic" and "Reason" - the latter is unfinished, I think) is amazing, like visiting the same place every few years to find how beautifully it changed. And this old level called "Rolly Scrolly" (or "Scrolly Polly"? seems like there were several versions; I'm talking about the one with 3 secret areas and 2 endings); a marvellous idea so well-realized that I played it many, many times after I completed it, simply to enjoy how perfectly it works.
There are also some "simply beautiful" levels, such as "Riku Island", and the unfinished "Island of Mimir" by Sinix, which is just stunning visually. I don't care much for the story in the first and the gameplay in the second, but they work very well for me nonetheless.
Then there are two authors in particular that have a very distinct sense of style, Farik ("The Hunt", "Harvest", "Ascension", etc.) and dessgeega ("The Lighthouse", "The Oubliette", "Torchlight", etc.). I don't particularly enjoy the latter's style, but its very, very distinct and well implemented. Farik's levels I always enjoyed. I hope he finishes "Homeland" one day.
Finally, although I don't like challenges or extra hard levels, but there's one that I enjoyed immensely, Klamrisk's "Room Ov Death". It seemed like hell the first time I played it (and at least one of the screens still does seem like hell), but I think its wonderful anyway.
(Sorry about ranting for so long
)