I saw this thread on the old old forums by Ice Eagle and now I'm going to bring this back up again, this time as a poll!

. You can vote for two sets here, but I'm not sure if you can vote twice on the same option. I'll check soon.
Edit: You can't, I've just realised. Anyway, I like Brick and Snow. Bubble has no metal, and Rock doesn't have many straight objects.
My two votes have gone to Brick and Bubble. The Brick style is very versatile, and even though it is overused, I do like the look of it. The Bubble set has some fantastic pieces in it. I'm not quite sure why they didn't put any steel in it, though.
Brick, and it's not even close really, at all.
Bubble is an incredibly distant 2nd, but no steel makes the set really hard to design with.
Brick and rock. I'd vote for bubble, but it has no steel

(yeah, that's enough to tip the balance IMHO)
Brick, for sure. The rest have an annoying lack of straight-pieces, making good levels annoying to design (although not impossible, see "Not Gonna Work" and "Return of the T", designed in rock and snow respectively).
I went Brick and Bubble, because they're quite versatile. Snow and Rock are too similar.
Brick and rock. I'd vote for bubble, but it has no steel
(yeah, that's enough to tip the balance IMHO)
One advantage to working in Cheapo

I voted for Brick and Bubble, and Snow is definitely my least favourite out of all nine, even in my Cheapo version with that horrible icicle trap replaced by a less gruesome one. (But I have still made some Snow levels that I like, such as "Vignette".)
Lots of votes for bubble here, but the dominator is the brick set.
I feel ashamed to be the only one who loves snow.* Snow is a bright, colourful set with perfectly suitable traps. Even when I was younger, Snow and brick were the only ones I liked because I hated those rock and bubble traps.
Hang on, I just saw another vote for snow.
You shouldn't feel ashamed. I think it's great when people perfer stuff that other people don't. It would be very boring if everyone liked the same thing.
I like Rock and Bubble the best.
Rock and Bubble? Interesting...
Well guess what, I've created so many rock levels (Why have I bothered?) that I might as well put them up on the archive, but they're all made with Oh No more lemmings graphics and my concern is when I put them up they turn to fire sets.
Those are mainly the ones I like designing in, actually.
Well guess what, I've created so many rock levels (Why have I bothered?) that I might as well put them up on the archive, but they're all made with Oh No more lemmings graphics and my concern is when I put them up they turn to fire sets.
Just copy and paste them into another Lemmix window in Custom Lemmings mode.
Bubble is cool, and Brick is overused for a reason (it's easy to make levels with). Bubble needs metal tiles though.
Bubble and rock.

EDIT: I don't know why I said bubble?
Definitely brick and bubble. Even rock is an excellent tileset, and snow is my least favourite, partly due to similarity with christmas tileset which was used in about 70 levels in Holiday and Xmas games.
Even though brick was the tileset of almost a half of the ONML levels, that doesn't bother me at all and it's the most epic graphics style. It was my favourite already when playing ONML as a kid and my favourite levels are mostly brick styled. However, brick doesn't own in all aspects: It's liquid is the most plain compared with liquids in rock, snow and bubble styles.
Bubble is my clear second favourite, it has unique, exotic world of its own. The acid in bubble style is a class of its own compared to any other Lemmings graphics style liquid. I'm little sad about that ONML had so few bubble levels (only 11 outside Tame......), and almost none of those were difficult (There were 2 bubble levels in Wicked and 2 in Havoc). "It's the price you have to pay" is therefore one of my favourite levels ever, even though it isn't highly difficult.
It would be very boring if everyone liked the same thing. 
Well, then we'd all be lemmings, wouldn't we?

Anyhow, my votes have to go to Brick and Rock, purely because I really like how they look. They're the ones that instantly struck me as being so different to the original Lemmings, for some reason, which is part of why I like them.
I don't design levels (I'm no good at thinking of puzzles

), so I can't comment on how well any of the sets work in this regard.
More users have registered recently so let's bump this and see if we can break this amazing tie between Brick and Bubble.

Remember, you can vote for just one set if you wish to, rather than two.
It appears that voting has closed on them... I'm not able to vote yet I haven't already voted. Same goes for the Orig one too.
Reopened this one as well.
My choices are brick (super versatile and nice looking) and rock (not a fan of bumpy terrain, but stuff in it just looks so pretty). Bubble is tricky to use (at least to make things look good) and lacks steel and snow is bumpy and a bit monotonous.
Thanks. I voted for Bubble and Rock.
I love the Bubble style because of its futuristic look and the traps in there, and I also find the bubble water to be very cool. I don't like that it doesn't have steel, but it doesn't stop me from voting for that style.
At first, I didn't like the Rock style, but I was young back then. Looking at it now, I am starting to appreciate the nature outlook of the style, and it is actually more versatile than you think, with the chameleon trap, the seaweed, and actually making unique designs with the branches (e.g. Friendly 12 in MobiLems, "Tree of Life", and a level by a past user who doesn't come here anymore that had the design of a chameleon using the grass pieces). However, that "OUT" sign on the exit flashes too much and it may cause a headache.
I voted for rock and snow.
Rock because it is a well made tileset that only experienced level builders can work well with, and when they do they can make good levels.
Snow because it's related to the Christmas tileset, but it still doesn't give me the Christmas tileset feeling, so I really can't make good levels out of it.
You should combine these two together and include the Christmas tileset with it.