I've always felt that Prelude in C major is Bach's prank to all of us. It's the start to Well-Tempered Clavier and makes you feel that maybe this set of compositions is approachable. And then you try out the fugue that's paired with this prelude and find that not only is it so extraordinarily difficult to master it at a technical level (your 2 hands have to play 4 different voices! And sometimes the middle voices require you to use your other hand for assistance), but also to be able to voice it well (your 2 hands have to express all 4 different voices clearly!).

Sheet: https://musescore.com/user/101554/scores/117422 Glenn Gould: https://www.youtube.com/watch?v=FUJpXpCbNgw

The the rest of the preludes and fugues follow a similar schtick.