Your favourite level and Why? (out of plain curiousity)

Some of the new / newly played levels:
plural - Hypnagogia - totally blew my mind. Unbelievably huge, non-linear and great all the way through. You gotta be prepared for the scale to play it though.
TJ - Cube(s) - are very cool, especially in that rear genre.
Raicuparta - Aperture Science
Raicuparta - Obey
Talps - Do Not Pick Up The Key
LordMarzog - Run the Gauntlet (both)
egomassive - The Dying Core
SecretGlitch - Dark Sea 2 - although the full version is still not finished.

Quite a large bundle of my favourites:

Monocromatica Grotta (B-Man)
The Great Race (B-Man)
The Lighthouse (Dessgeega) I don't often favour environmental levels at all, but this managed to tick the boxes somehow.
Shipwrecked (Egomassive)
The Dying Core [Beta version] (Egomassive) - Just to be awkward .
Venice Issue (gedzior84)
Return to Riku Island [Beta] (Kohdi) - Those few screens are beautiful.
A Strange Dream (Nifflas)
An Underwater Adventure (Nifflas)
Gustav's Daughter (Nifflas)
Skyflower (Nifflas)
This Level is Unfinished (Nifflas)
My Loving Hadron Collider (Talps) - I know, how arrogant am I!
Petals of a Dying Flower (Talps) - I just know what I like!

On the whole, I tend to prefer small levels I can dip into and play within ten minutes, to satisfy an itch. But some of the above are really exceptional.

Here are my favourites, though this list may change soon-ish:

-All of the Nifflas levels (Isn't it obvious?)
-Absolute Machination, by Diago (Wonderful atmosphere)
-Cliffhanger, by Grimwit (Not sure why I love this one so much, but I do.)
-Dark Sea 2, by SecretGlitch (Oh Juni, this is hard, but very VERY well made!)
-Do Not Pick Up The Key, by Talps (My banner should have given that away )
-Harvest, by farik (Solid story and progression.  All you could ask for, really.)
-Night 2, by Chezzy (I'm sure we've all been there, for one reason or another!)
-Scarlet Tears, by James Vaughan (The atmosphere for this one gets me every time)
-Station Sputzna, by farik (Even incomplete, this level is gorgeous to both the eyes and ears)
-The Extinct Bird, by Christian (The COs in this must've taken FOREVER!)
-The Hunt, by farik (Very interesting effects and settings, and decent humour as well)

-Monocromattica Grotta(B-Man) - Very good gameplay.It's fun to play even after the first time.Only level I've played just because it was fun.
-Sohe and the Rrokked(BaronCid & Old_Gil) - Very atmospheric world.The rocketship and the logbook are amazing.Only level where I really want to read.
-Night SE(Chezzy) - It's limbo time ! Good use of a concept
-Fossil(Dessgeega) - Great gameplay supported by fitting graphics.
-The Oubliette(Dessgeega) - Great gameplay again
-Torchlight(Dessgeega) - Great concept and gameplay AGAIN.Not a big fan of the beginning though.Doesn't fit in with the rest.
-Under the Crack(Dessgeega) - Best complete level for Knytt Stories for me.Simple works.Love the colours.Love that is had a good icon, good intro screen and simple start and end screens.
-Shipwrecked(egomassive) - Good use of shifts.Love the first days.Fighting static pirates is not my thing though.
-Uthoha(Elder) - Like the colours and that it reminds me a bit of Knytt
-Ascension(Farik) - Love the simple, but eyecatching colours and the difficulty does not make the game frustrating.Also a complete level.
-Camping(Farik) - Good concept.
-Deserter Life(Farik) - Short, but it offers some eyecandy.
-HomelandBETA(Farik) and Sputzna(Farik) - Although this whole epic theme that he has going is not really my style, I still get sucked into the world.Love it.
-The Hunt(Farik) - Really like the pacing in this one.And that is has some very imagintive creatures.
-Temple of the Sun(Fegon) - Again it's simple, it doesn't overuse a billion different colours.
-Knytt 8bits(Jerom) - Very good concept.Love the grayscale.Although it is a bit glitchy and I did get stuck.
-Apeture Science(Raicuparta) - Great tribute.
-Chasm(Sergio Cornaga) - Love the warm tones.I really wanted to explore every bit of this level.Very enjoyable.
-Petals of a Dying Flower(Talps) - Simple colours and good gameplay.
-Starlight(Vonicles92) - Everything fits so well in this playground level.Soothing.
-SplitMind(Jigganis) - Awesome use of the old tilesets.

I prefer easy and simple levels with satisfying ending, because it helps me relax during my schoolwork. I also like enviromental levels, but I can't play them too often - they begin to bore me.
So here is my top list:
I am not a mad scientist by Pr0xyzer - it is short level with several mistakes and very simple story (and first mistake could turn many people off playing it - it is uploaded in the archive under twisted names of author and level), but I like it.
Gaia by Anyndel and Drakkan - sooooo big
Extinct bird by Christian Schellmann - I love this original enviroment.
Harvest and Hunt by Farik - my favourite levels by this author - but I like other ones too.
Torchlight and Under the crack by Dessgeega - my favourite levels by this author - but I like other ones too.
Dark sky of wish mountain by AlmightyZen Taco - one of the first levels I've played
Deserted rustyard by Bligglenuber - these COs always make me wonder
Falling water, Jumbping in the rain and Remembering the city by Hmpf MacSlow - I love them all and want another part of the first one
The dark witch by Nachtom - Nothing special, but one of the best Czech levels (there's an English version in the archive).
Skyfalls by Pip - everything works perfectly
Marine by Jerom - beautiful steampunk tilesets and good story
Petals of the dying flower by Talps - just perfect (and about flower)
Well of white waters by Headgrinder - beautiful place. Why is it not bigger ?
Grim, Scorching the sun and Parasite trilogy by Strange darkness - brilliant horror atmosphere
Lost in storm by Razzorman, Natural bySnawnachu, Parkour by Sergio Cornaga,
Outer reaches by Pick yer poison and Mashu prapa and Reverie by Elder/Salmonelous - my favourite enviromentals.

Edit: I have to add two levels I adore and wish to be able to make something as good as they are. I've finally finished them:
Hypnagogia by Plural and I dreamed of the sky by Talps - both are just perfect.

Here's some of my favourite levels and why:

I'm so cold... by Bass - it's a simple but funny level with loads of different endings. Note: funny.
Endless Time by BloxMaster manages to conjure an epic feel. It's epic.
Night by Chezzy makes the epicness of being out alone at night come to life.
All levels by dessgeega. Artsy-farsty and epic-peppic.
Romance by Elder. It's just insane, but maybe still true, in a cynical way...
Core by Fredrik Andersson is maybe my favourite level. Everything just FITS.
Capitalism by googoogjoob is everything I don't want my life (or the world) to be. "An interactive loss of will to live" - you don't say...
Splinter by Iggyzor. Another fun meta-level.
Marine by Jerom. A huge and sometimes confusing level to get lost in for hours. Last time I played it one of the shifts didn't work out though... I'll try to find a new version then! 
splitMind by Jigganis. Interesting philosophical monologuing.
Teenhmifnoeafgil & Too Much Soda by KG. So unnecessarily over-thought-through. So undeniably awesome. (And lovingly weird)
The Room Ov Death by Klämrisk 
A Psychedelic Adventure by Medic-Ted. A huge and sometimes confusing level to get lost in for hours. Last time I played it one of the shifts didn't work out though... I'll try to find a new version then! 
Jolio episode 1 & 2 by Noi. On the subject of fun, stroy-driven, really worked-through, random, high quality levels...
Rememberance by Si. Yes. Just yes.

Marine by Jerom. A huge and sometimes confusing level to get lost in for hours. Last time I played it one of the shifts didn't work out though... I'll try to find a new version then!  

I'm pretty sure someone fixed that problem and posted the updated version on the current forum somewhere... I'll have a look around, see if I can find it for you.

Edit: Ah, it was GrayFace, enjoy!

Donwloaded, will play later. Thanks! 

Too Much Soda by KG

Every time I play that game, I pretend that I live in one of those huge houses in the beginning. Imagine all the stuff one could fit in something that big, man. Just thinking about those houses is really great, you know? Forget the story, characters, and the love KG put into this game, it's all about those houses, haha. Oh man, I think I'm going to go play that again and relive my dreams of owning a giant house haha.

Too Much Soda by KG

Every time I play that game, I pretend that I live in one of those huge houses in the beginning. Imagine all the stuff one could fit in something that big, man. Just thinking about those houses is really great, you know? Forget the story, characters, and the love KG put into this game, it's all about those houses, haha. Oh man, I think I'm going to go play that again and relive my dreams of owning a giant house haha.

Nominated for best reason for a favorite level.

I love Too Much Soda. It's the funniest level I've played.

"Infestation" without a doubt!  And all of Nifflas's originals of course.

I am reviving this thread mainly to post an updated version of my favorites list, and also to get people to list off their favorites again!  For fairness, I'll go with one level per designer.

Here's mine:

• Gaia by Anyndel and Drakkan:  The greatest in scope of all environmentals.  You'll spend quite a while exploring this world.
• Sohe and the Rrokked by BaronCid and Old_Gil:  Great level with just the right amount of creativity and confusion.  Too bad it isn't quite finished.
• Deserted Rustyard by Bligglenuber:  Masterful use of atmosphere and COs.  You'll feel like you are playing a completely different game!
• Night 2 by Chezzy:  A classic scenic level.  Play with the lights out.
• The Extinct Bird by Christian Schnellmann:  An excellent example of atmosphere and use of COs.  A lot of effort was obviously poured into this little level.
• Nameless by Cosine:  Never has utter emptiness felt so compelling, and harrowing.
• The Oubilette by dessgeega:  Another great mood-setting level, with nice challenge.
• Oil by Diago:  A fine example of a level that sets an inviting mood.
• Sputzna by Farik:  Still unparalleled in atmosphere, despite being unfinished.  I'd also highly recommend Farik's other levels.
• Cosmic Meltdown by Fubaka:  Yes, I'm putting my own level on this list.  I honestly think it's a great level, with a very unique way or story-telling.  It's also the single biggest level I've ever seen as of yet.  (Yes, even bigger than Gaia, Teenhmifnoeafgil, and Frozen Mountain 2!)
• Cliff Hanger by Grimwit:  Still no idea why I love this level so much.  Maybe because of its sheer surreality.
• Awful Mountain Restaurant by Healy:  This level is an ironic pleasure for me.  It makes me want to do something like it in real life.
• Scarlet Tears by James Vaughan:  This technically isn't a 'good' level, but it has appeal to me in the same way Frozen Mountain 2 has for others, at least in my opinion.
• Teenhmifnoeafgil by KG:  On the surface, this level is nonsense, but as you explore further, you will find perhaps the most unique and fascinating experience a KS level can offer.
• A Strange Dream by Nifflas:  Definitely my favorite of the Nifflas classics.
• Endless Winter by Nifforum:  A wonderful collaborative effort with great atmosphere.
• Orientation by plural:  The ultimate in presentation, and stunning use of COs.
• The Tree of Life by Purple Ink:  More lovingly crafted than most any other level I know, and a foremost in exploration.
• Dark Sea 2 by SecretGlitch:  The perfect difficult KS level.  That's all I have to say.
• Juni Jones: Ace Attorney by Sergio Cornaga:  The perfect terrible KS level.
• I Dreamed of the Sky by Talps:  A level truly deserving of the 'Epic' title.  I'd recommend checking out Talps' other levels too.

If I were to pick my top 5, they would be as such:

I am reviving this thread mainly to post an updated version of my favorites list, and also to get people to list off their favorites again!  For fairness, I'll go with one level per designer.
...

Thanks for the list! I'm always looking for new levels and there's several on there I hadn't heard of.

Also, Cosmic Meltdown was really that big??? I guess I REALLY need to replay it and get all the keys! (I got the eye though) It didn't feel bigger than, say, Eurydice or Dark Sea 2 but I guess there's still a lot I missed.

The more I learn, the more impressed I am with it as a level. So many layers!

Don't Eat the mushroom
Awesome psychedelic trip, and the endings are pretty funny. The only downside is it's only about 10 minuts long