Having finally finished them all (getting all coins and bonus coins), here are some of my own comments on various levels (excluding mine), separated by different criteria (levels listed in no particular order):
Good puzzles [as in, "how hard did it, um, exercise my brain?"
]:
A Small Adventure, Stinky Crusoe, Just 3 Coins
Good Visuals [as in, "it looks neat!"
]:
Stinky Crusoe, Surprise, Lightning Strikes, and of course, Space Filling Curve
(though I have to admit I didn't see the 14x14 in Lightning until it was pointed out)
I have a little troubling liking the combination of psychedlic color-cycling on the "Garden" style as used in "It's a Contest" and "Jailbreak".
Good Organization [as in, "doesn't look too much like someone didn't tidy up their room in a week"
]:
More than Meets the Eyes, It's a Contest, Just 3 Coins, Stinky Crusoe, Laughter, Lightning Strikes, Simple Plan, Surprise
More than Meets the Eyes, in particular, stands out in my mind as the level that has the best balance of variety and tidiness.
I guess I should include CoinSearchers and Space Filling Curve, but it's rather hard to make those look cluttered even if you try
.
Adventure Land 2.5 is a bit too cluttered, probably because the concept of Adventure Land style levels is hard to cram into a 14x14 level.
A Small Adventure is also leaning towards the cluttered side.
Compactness [as in: "how many different ideas did they cram into the level?"
]:
Stinky Crusoe, A Small Adventure, It's a Contest, More than Meets the Eyes
Stinky Crusoe probably wins compactness hands down; what's most remarkable to me is that it doesn't look as involved as it seems, but when you try to solve it, you quickly realize there's a lot more to the level than your first impression may suggest. A Small Adventure did pack a lot but as I said in "organization", it shows visually.
Creative New Ideas [as in: "hey, I haven't seen this before!"
]:
(warning: this may give away how to solve the level)
Adventure Land 2.5: need I say more? As it turns out I missed the connections between the various objects in the level with the backstory. Now it all makes sense, and that alone made the level much better.
I especially commend Robbie's effort to push the game engine to its limits, in order to explore possible new directions for the game. In that sense, he may be the most creative of us all.
While the end result has its imperfections (and really, part of this is only because the existing game engine is not flexible and versatile enough), it still makes for a pretty interesting level to play, as long as you pretend there are certain things you can't (ie. not supposed to) do that the game engine lets you do anyway.
Stinky Crusoe: "kill the steel box with a gate", "the Kaboom is your friend" (and the convoluted way to get it to become your friend), "don't let that steel block come off the conveyor belt"
Just 3 Coins: the placement of the blue teleport (the one near the cannon) is quite nice--not easy to see unless you're careful
More than Meets the Eyes: the steps needed to activate the pink button is a nice puzzle, although I might've seen it elsewhere. And the use of fake walls, although I'm sure it has occurred in many other levels, still feels particularly fresh on this level
Laughter: although doesn't add a lot to this level, the idea of placing coins on a teleport is interesting; it often requires one to go thru the teleports more than you would otherwise, which can definitely lead to some interesting puzzle ideas
Lightning: the first part, 'Nuff said
(And the 14x14, and hatted cannon are nice touches as well)
Space-Filling Curve: need I say more?
Surprise: the element of surprise is
extremely well implemented here
And last but not least,
Fun! [as in, "who cares about the other stuff I just said?"
]:
(also partly measured against by the amount of profanity induced
)
A Small Adventure, Just 3 Coins, It's a Contest, More Than Meets the Eyes, Surprise, and of course, Space Filling Curve (sheer fun factor there!)
Stinky Crusoe hurts my brain (and so many places to make a mistake) just a little too much...I guess that must be what my level's like to everyone else, hehe
Lightning did have a somewhat high frustration barrier, but wasn't too bad once I got past it
Jailbreak is kinda fun the first time I played it, seeing how all those monsters get killed off randomly one by one.
(the other levels were great fun too, just that the ones I listed here stands out just a tad more for me)
.... can't believe I fell for that when I myself used that trick in the "What's in That Hedge?" level 
). 
- that is sounding like a better solution all the time!
But, I REALLY want to solve it!!!
