I'm still a bit inordinately pleased about the 100% solution for Taxing 18. Again, probably one you all know. It seems to be easier on the Genesis version, because the gap between the exit and the wall is wider...
And actually just about completing Havoc 16 and Mayhem 26 at all on the Mac version, since they have graphical bugs that actually affect gameplay. I've attached a screenshot of my solution for Havoc 16 to try and illustrate: essentially, the little wall near the top is perfectly straight in the Amiga/DOS versions, but on the Mac version it's a bit bumpy in such a way that a climber can't get up, but by laying a step as soon as he lands on that pixel, he'll climb up properly the next time he goes up there. Annoying as hell, basically.
I also prefer levels with a challenging puzzle that is (relatively) easy to execute. But inevitably it's the other kind that'll stump me for longer. It was like that with Present 29 recently – I only managed to beat it the other day with the help of savestates.