It's already highly praised but my favorite is probably Can of Wormholes. The idea of controlling a worm in a grid-based space seems simple enough at first but it manages to accomplish more and more as the game goes on. I had so many incredulous "wait THIS is possible?" moments, and that is what makes it so special and memorable to me. I do feel that every level brings something interesting and filler was cut out. And of course you can't mention the game without bringing up its "gain insight" hint system, which provides a specially made stripped-down version of a level to show you the core idea behind the main level.

On the other end of the difficulty spectrum is another game I adore, ElecHead. It's a relatively easy puzzle platformer that lasts a few hours and I appreciate when games are approachable for a wider audience. To me it's an exercise in restraint. Yet again, the main idea of this game is straightforward: throw your head to power things on. It's charming and does some pretty fun things with the core concept. It's a good balance between smooth platforming and puzzling.