Like Kaywhyn, I found Tricky 3 "Which Way, Mr Scarecrow" the most difficult level – it took me a long time. Each lemming has to navigate 3 bi-directional splitters to get home, so – even ignoring the Zombies - there are 8 potential paths, of which 2 need no intervention, 4 need one, and 2 need two. So 8 lemmings would need 8 skills, and 20 need 20 – which leaves only 5 skills free for evading Zombies. I suspect that is impossible, so some lemmings need to be helped by either a Zombie or an earlier lemming turning the scarecrow in the desired direction. I looked for a pattern with the first splitter encountered: alternate left and right, or alternate in groups of 2, 3 or 4? Unfortunately I tried 3 of these options before finding the one that worked. With the help of 92Dexter11's tip about turning lemmings around, I eventually found a solution that uses just 23 skills.

Treacherous 3 "Pumping Station" initially looked like one of those levels where the required strategy is clear but execution is time-consuming and tricky – having to send up a succession of perfectly-timed Climbers to make sure that later-spawning lemmings do not splat. But it turns out there is a solution that avoids any risk of these splatting and only needs 2 Climbers (maybe only 1, but that would make timing very tight).

Treacherous 6 "Pushing Up Daisies" is ingeniously designed and provides quite a challenge for the Talisman. But it's one of those levels where you can get close and then do some final tweaking to avoid running out of Platformers.

Treacherous 10 "Ring Of Fire": a nice challenge to end on. I was having trouble rescuing the lemming that acquires the Jumper pick-up without causing problems elsewhere, so I looked to see if there was a solution not requiring the Jumper. And there is.

Fun: early levels provide useful introduction/refresher for skills used later on. I found a couple of possible backroutes, for levels 10 'Laugh if you like lemmings' and 25 'Lemmings, lemmings not everywhere'.

Tricky: Backroutes for levels 6 'Lemming psychology' and 26 'She has a clever plan'; maybe also for a couple of other levels.

Taxing: I particularly liked the puzzle provided by level 11 'The ascending shimmier scenario,' where  my solution is a minor variant of the author's, which saves a lot of time; also 28 'Poor lemmings.'  The hardest level for me was 26 'Trouble times 3' where I found it very difficult to synchronise the timings. My solutions for quite a few levels may be backroutes – notably level 2 'Many traps about'; level 12 'On the Edge'; 15 'This is an AWESOME level;' 16 'Heights beware'; and 25 'Follow the boss.' For level 7 'Every lemming must think for himself,' my solution is faster but far less fun. Level 30 'We might all fall down' is another level where a slight tweak to the solution can save a lot of time.

1 Blue Works: a lengthy but relatively straightforward level, with what seems to be a major backroute available.
2 This room is boiling: the most difficult level in the pack, for me. There are so many possibilities as to where to deploy the Cloners, and it is tempting to use the first one too early. Also, I wasted a lot of time trying complicated ways of accessing a couple of the pick-ups, because I failed to spot the easier way. The Zombies on this level are a minor distraction that just delay the effective start.
3 Be a hero: I reckoned there was only one way to solve this within the deadline, but I was wrong – my solution differs from the author's.
4 The roads that cross: 3 sets of lemmings to be co-ordinated. There must be lots of different solutions here; mine is reasonably quick. Whichever one you go for, you have to be patient with those on the right – possibly even the advance party. 
5 Up and down!: again there must be quite a few solutions available, at least in relation to accessing the pickup skills and creating a path for the upper group. 
6 This way or that: a relatively easy level for this rank, as I could not see any alternative solutions, apart from minor tweaks. Timing is tight though. 
7 Separated poles: not too difficult a level; again I would think there are several alternative solutions possible.
8 Don't be a rotten egg: timings are tight again here, but there's still some scope for tweaking the start and exactly how the Cloners are used. 
9 The Pharoahs curse: this is a very long level, in space and time. 3 trapdoors, 3 exits, buttons and teleporters make it quite a saga.
10 Hercules'pillars: not quite as difficult as it looks at first sight, but still a considerable challenge. I looked for a route that did not use so many Stackers.
11 We mostly all fall down: a breather level. With a locked release rate, there's no chance of beating the time limit by much. 
12 The side too far: another long level, with the advance party doing all the work.
13 The lemming thief: a nice little puzzle – only 2 lemmings, but lots of buttons to be pressed.
14 The split pea soup: a range of paths available, but good co-ordination needed whichever one you choose.
15 The quicky food solution: a close study of the skills available allows you to work out what the solution must be – I doubt if there are any alternatives to the main part of the solution. I ended up with just a fraction of a second to spare.
16 Only a minute: another easy breather level.
17 Don't step on stones: a good variety of solutions possible here, and an interesting use of a cascade of splitters. I had to work hard to find a way of speeding up the final lemming, to stop it lagging a long way behind the others. It's possible to beat the time limit by nearly 2 minutes, though that solution is not very elegant.
18 Only 4 of them: a very difficult level. In the end I came up with a crude solution, which meets the 'Save' target with over 1 minute to spare, but feels like a bit of a cheat. However, one of the pleasures of the game is seeing how many different solutions there can often be to any particular challenge.
19 Up you go!: one of the less interesting levels – straightforward, lots of platforming and bridging.
20 Discolored lemmings: it took me a while to work out how to access the pickup skill embedded in the ceiling. The vast array of traps before the exit means that the group cannot be released early. It's obvious where the Cloner has to be used, but there is scope for varying exactly how the Shimmiers are used.
21 A lemming twist please: a frustrating start, as you have to wait ages for the Neutrals to collect pickup skills. After that, it's an interesting challenge to work out where to deploy the Cloners.
22 A Beast!!! of a level: a key part of the challenge here is reaching all the buttons required to open the exit – quite a bit of trial and error is needed, along with some tweaks to stop a couple of lemmings heading away from the exit.
23 Up we go!: a very cleverly designed layout. Some aspects of the puzzle are easy to figure out, but I really struggled to find a way of freeing the lower group without causing a problem elsewhere.
24 All in please: fairly easy to work out what has to be done, but the timing is very tight.
25 Have a nice night!: the location of the teleporters and pickup skills gives useful clues to deploying the Cloners, and other key aspects of a solution are relatively easy to work out. But there is still scope for finding a speedy solution.
26 Steel areas of K*****: a key aspect of the challenge here is working out how best to access all the buttons, and in what order. How you use the Jumpers is important. A lot of trial and error got me to a speedy solution.
27 Another minute: a very easy level – maybe more appropriate to the Tricky rank? 
28 Don't mind the step: another long level; a bit of time can be saved if you find a way to avoid the Platformers reversing each time they bridge a gap.
29 Save me once more: I love a well-designed Zombie level, where you have to figure out a way of containing and/or removing the Zombies. My first attempt involved holding them back at the 1-way tower, but I couldn't get the timings right. In the end I came up with a tighter containment, which looks like a backroute. 
30 Meet in the middle: a nice puzzle to finish on. I could have done without the long wait for the Neutral to collect vital pickup skills. The layout and skillsets allow you to work out exactly where the Cloners must be used. But you have to work out how to use the teleporters; and it took me a while to realise where the Stoner and Stackers have to be deployed.