These are what we call the 'YMMV items.' Things that some people find in this work. We call them 'your mileage might vary' because not everyone sees these things in the same way. This starts discussions in the trope lists, a thing we don't want. Please use the discussion page if you'd like to discuss any of these items.
Best Level Ever: It's easier to list the ones that aren't a joy to play through.
Big Lipped Alligator Moment: The disco party with the Magician at the end of the game. You just fought your way out of his Death Trap to come back up to the control room, and instead of running away or fighting, he starts dancing...and Rayman inexplicably joins in.
Breather Boss: The Mockingbird, even though he's only the 3rd boss. His patterns are extremely simplistic, there's no stage to go through before him, and its phases are very short. With a Difficulty Spike boss before him and a boss with lots of Trial-and-Error Gameplay and a long, annoying stage after this boss, he's notably easier (although there's no specific order to fighting the bosses)
Fridge Brilliance: In the first trailer, "the second summer solstice" sounds like a nonsense joke until you remember that the Glade of Dreams like Tatooine has two suns.
Rayman dancing with The Magician at the twist is also as ridiculous joke, until you realize that back at the first Rayman game for the PS1, every boss you defeat than starts dancing to show that they befriended Rayman. Looks like The Magician seemed to pick up on that as well in order for him to escape...
God Damned Bats: Pufferfish. So damn much. The timing to hit them is off, and it will take up to 5 seconds for them to retract the spikes again.
Nightmare Fuel: Most of "Swimming with Stars" since it's the first level that isn't whimsical and cheery like everything else up to that point. If the creepy enemies didn't get you, then the oppressive darkness and haunting music will.
Shocking Swerve: The Magician being evil comes out of nowhere and with little to no foreshadowing, granted, there is a reason in the beta script, but that isn't in the game now, is it?
Some of The Magician's comments when you bop his top hat foreshadow his villainous qualities, or at least that he's not nearly as nice as other characters.
That One Sidequest: By far, the worst Tricky Temple levels are easily the fourth and sixth. The fourth in particular is extremely trial and error heavy and ends with you having to wall jump up wooden pillars with fires that can screw you over if the scrolling camera doesn't, but the sixth one-ups it because at the very beginning there are spike hazards at the perfect height to thwart 9/10 of your attempts to jump between them.