Wow, it seems that in my short hiatus from reading the forum I missed a new peak in activity, the start of a level designing game and the return of Insane Steve!
Anyway, here's my somewhat belated evaluation on the best and worst levels:
Best in Original Lemmings
Interestingly enough, I feel quite the opposite compared to ClamSpammer in regards to Original Lemmings and ONML: I rather find many of the original levels pretty uninspired (Genesis excluded: It seems like the level size limitation was a blessing with many horrible levels being replaced by interesting ones).
However there are a few levels that stand out in the (your mileage my vary) sea of mediocrity. I compiled these few together with my favourite levels from the Genesis version (which happen to occupy the top spots) into my personal Top 10:
10) (Genesis) Present 29: Private Room available: Pretty unique in its conception, despite only a few additional skills, the solution isn't immediately evident after knowing the technique.
9) Mayhem 3: It's hero time!: Nice 1-of-each-skill puzzle with a variety of solutions.
8 ) (Genesis) Sunsoft 22: Watch left or right (Part Two): I'm pretty certain there are multiple solutions to this one, but none is visible at first sight.
7) Mayhem 27: Just a Minute (Part Two): A different kind of goal compared to most other levels, requiring some thought to achieve.
6) (Genesis) Mayhem 29: No hurry, Relax.: The solution isn't hidden too well, but it's pretty elegant.
5) Mayhem 13: The Great Lemming Caper: Apart from its many backroutes (could do with some skill-reduction), it's a nice two-lemming puzzle.
4) Mayhem 20: No added Colors or Lemmings: A fair introduction into the 'undermining' mechanic, and for me the best puzzle level in Original Lemmings (Amiga/DOS).
3) (Genesis) Sunsoft 30: Fall and no life (Part Two): Even though it has a couple of solutions, neither of them is trivial, and what I suppose to be the intended solution is pretty nifty.
2) (Genesis) Sunsoft 29: I am A.T.: A classic emplying a very unique technique which has inspired quite a few fan-made levels. Has it ever been found out who the mysterious 'A.T.' is?
1) (Genesis) Mayhem 30: Lemmings' Ark: Magnificient level, beating the entirety of Lemmings and ONML. It took me an entire week to solve this elegant puzzle. Sadly there's one (or a few?) bad backroutes to it, but they don't prevent you from looking for the intended solution, as they are too unaesthetic to be satisfied with.
The next two in this list would be yet another two Genesis levels, Mayhem 1 (Let's get together) and Taxing 20 (Fall and no life).
Worst in Original Lemmings
I don't really have levels I particularily learnt to hate because I just skipped over the tedious ones until Lemmix made them bearable. Let me just list the two I remember in particular, as otherwise this list would get endless:
Mayhem 1: Steel Works: When I was still playing the original lemmings in DOS, this always put me off to get started into Mayhem, because it's just so unnecessarily long and boring.
Taxing 14: Hunt the Nessy....: The epitome of mindless tedium. Sadly, it is representative for a lot of levels in Original lemmings: Get from one point to another using a worker lemming and a vast supply of skills (usually builders).
Top 5 in ONML
5) Wicked 3: This Corrosion: Not overly difficult, but a clean and elegant solution.
4) Havoc 16: Scaling the Heights: There are various ways one can think of setting up the bridges, however the timer requires you to optimize. Sadly backroutable.
3) Crazy 10: ROCKY VI: It's pretty clear where you need to go, but fine-tuning is required to get along with only 8 builders.
2) Havoc 4: The race agains cliches: Despite being a somewhat long level, the are multiple interesting solutions neither of which is trivial.
1) Havoc 5: There's madness in the method: Objectively it's likely not the best one, but back when I wasn't quite as good at the game, I remember myself being mesmerized by it because of how clear it looked, the very limited supply of skills and yet me not being able to find the solution for a few days.
As for the worst, again there's none I hate in particular, I'd just go through and note the tedious ones again (which frequently happen to be snow styled levels, for some reason).