All rightie, after 5 months the Sun rank has finally fallen. Replays attached up through the Sun rank. Up to this point in the pack, I have to say the most difficult levels so far for me were Planet 13 - 50% is not enough!, Sun 7 - Scrap the Builders, Sun 12 - Death of a Working Lemming and Sun 20 - Empty Walls, although the last two were nowhere near as hard as the brutal Sun 7. I thought both Sun 12 and Sun 20 were hard due to the fact that the solutions in each one are difficult to spot, with the latter the way more difficult one. Also, I thought that the levels after the extremely brutal Sun 7 were much easier, although I guess the level immediately after, Sun 8 - Minimalistic Platforms was somewhat hard, but since there's only 2 different skills, a blocker and 5 builders, it's tricky more than difficult given the timing.
In particular, the 1-of-everything levels in this rank, Sun 4 - Just a Quickie!, Sun 11 - It's Hero Time Again!, and Sun 20 really stand out as being difficult, with Sun 4 probably the most difficult of the three due to the extremely strict time limit, only 30 seconds. Without it, it's a really easy level, but the time limit really ramps up the difficulty considerably. Of course, in Dos and Lemmini this isn't a problem, as the amount of minutes can only be an integer, which means it has to be given at least a 1 minute time limit in those engines.
I've noticed that a lot of the levels in the Sun rank were small in size, being only about a screen wide, but they were still hard levels. It goes to show that small levels may look easy in appearance, but they can be surprisingly hard. Indeed, don't expect to solve through the pack very quickly! The levels get hard starting with the Planet rank, and they get harder really fast. The Planet rank is the first rank after the X-of-everything levels in the Comet and Moon ranks, 5-of-everything in the former rank, 10-of-everything in the latter rank. Even the other levels I didn't list were challenges, and my progress slowed considerably starting in the Planet rank.
In any case, after I solved the Sun levels on my own, I compared my solutions to those of Icho's on Youtube. I find it quite amusing that in almost every level in the rank, I was able to save a skill(s), while Icho used all of them. For example, in Sun 9 - Silence on Earth, I have a blocker leftover, Sun 11 I have a floater leftover, Sun 13 - Cannonballs in the Air I have a bomber leftover, Sun 15 - Revival of the Lost I have a blocker leftover, Sun 16 - The Marble Temple I have two miners leftover, and Sun 18 -Five Survive, I have a climber leftover. That's 6 levels where I did better in skill usage. I swear, I wasn't even trying to be more efficient in my solutions than Icho was. I couldn't be, since I only watch solution videos after I beat the level first, so I was quite surprised I ended up saving skills on many levels compared to him in the Sun rank. On the flip side, he was much better in skill usage than me on both Sun 10 - We All Dig Down! and Sun 19 - Just a random heap of junk!. I especially love his solution to Sun 7. It's definitely the easiest and cleanest solution I've seen to the level which escaped me completely. I think Sun 20 was the only level after Sun 7 where we had the exact same solution.
Finally, Icho's solution to Sun 4 might not work (key word: might) in the current NL version, since lemmings no longer can exit when the time limit runs out. In it, the last lemming exits exactly when the timer hits 0. So, I got to say that my solution is more efficient in that I have 3 seconds left on the clock, although it's not a trivial solution at all. Also, in Sun 12, I managed to save an extra.
edit: I just watched Icho's replay. Yup, the timer hits 0 at the exact moment the last lemming finishes exiting (easiest way is for the last lemming to exit and then use the N key until you hear the ringing alarm), but the game still gave him the credit for passing the level. Wow, imagine if it was one frame too late instead.
Well, onto the Neutron Star rank, the second to last rank of the pack. Already nervous about what the rank has in store, particularly if the Sun rank was any indication. In particular, every level up to Sun 7, inclusive, were all difficult and tricky, while the levels after Sun 7 generally got a bit easier, although not by much. Here goes nothing!