I feel like I'm "digging up" an old topic here, but I guess, it's a kinda timeless question
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I'd say "fun" is not just a matter of a specific level, but rather of the sequence of levels in succession to each other. No one likes doing the same stuff over and over again. Take for example "Don't let your eyes deceive you", "Don't do anything too hasty", and "Take good care of my Lemmings". They are effectively all just about trapping the crowd between blockers (or, in one case, pillars) and building a long, long staircase to the exit. Especially original Lemmings was a real Builder-fest, and back then without space bar skipping or fast forward, I feel like 90% of the time playing Lemmings was watching a Builder doing his work and waiting for the click-click-click sounds to start the next one ^^.
Lemmings 2 shook things up, but in my view it went so completely over the top that it rather feels like playing WORMS - with the Lemmings getting tossed around by knockbacks all the time, plus it has some typical WORMS skills, like jetpack, bazooka, mortar etc. The much more important skills from The Tribes in my view were those literally "filling gaps": Fillers / Glue Pourers, Platformers, Swimmers... all this stuff you'd have to use a builder for in original Lemmings. Also the possibility to use destructive skills in different directions, like the Fencer does, was a welcome addition for me.
Hence, in my own levels, I like to mix things up between Builders and Platformers, Miners and Fencers, whether one has to hold back the crowd with a Blocker, Stacker, or Stoner, or with a Basher/Miner/Digger pit. Whether one has to protect Lemmings or can afford to sacrifice some despite having to save 100% - because Cloners can compensate for that.
In short: NeoLemmix gives us a lot of tools to create versatile challenges. If I'm facing five levels in a row where I sit before a paused screen for hours until figuring it out, that will demotivate me quite quickly. One level might be very "puzzly". The next one may be pure reaction skills ("All or Nothing") or stamina ("We all fall down"), or a builder-fest. Just keep altering between the type of demands each individual level poses to a player.