I love this question! This made me really happy. My favorite has always been (and remains) the brontosaurus. (Probably can blame “The Land Before Time” for its influence.)
I remember in the early/mid-90s being corrected that brontosaurus wasn’t a dinosaur species, but was actually the same as apatosaurus. Being a stubborn child, I refused to accept this and always considered the brontosaurus, not the apatosaurus, my favorite. I felt so vindicated when the study came out in 2015 that brontosaurus and apatosaurus weren’t the same.
Perhaps that’s why I still stubbornly refuse to let go of Pluto: the minute hope that my favorite planet will be recognized again someday. Alas.
The Princess Bride is one of my favorite examples of this, especially because the “story within the story” is the main story, which is unusual.