You might be expecting me to exclaim about Martin the Warrior from the Redwall series or Lessa from the Dragonriders of Pern. Based on the title, that's the sort of thing you might reasonably anticipate.
But I'm a silly person who enjoys subverting expectations.
We're talking about my favorite character type. Not necessarily the one that's easiest to like, but rather the type that's the most fun to interact with as an author.
Recognize this glorious disaster of a person? Perhaps you don't. This is the famous Mr. Willy Wonka (of Chocolate Factory fame, played by Gene Wilder (who is the only REAL Wonka)) and as you might recall, he is NOT the main character of that story.
The book, after all, is titled Charlie and the Chocolate Factory. But not many of us read that book for the sake of Charlie Bucket. We read that book (and watched those movies) for the sake of Wonka and all his craziness.
Or how about this one?
One could reasonably say that Holmes is the main character of that story, as it's his name on the title, but he's not the one that's in every scene. (This is Rathbone here, if I'm not mistaken, though I have a weakness for Ronald Howard from the old black and white TV series.) We don't read (or watch or listen) to The Adventures of Sherlock Holmes for Watson and his commentary. We watch (and listen and read) for Holmes himself .
There are others, too, like Phileas Fogg, from Around the World in 80 Days. These characters are (in general) not the main character of the story in question. They are not the characters that are moving the story forward, so much as making the story interesting. They are the main attraction, and very near the core of the story, but they are often not the main character, because the story isn't about them. There are a couple of the Sherlock Holmes shorts and more than a couple of the television adaptations that are definitely not about Holmes - they're about Watson (see any episode ever where Watson is mad at Holmes for being a social lead weight).
So you end up with these main characters that are witnesses to the thing we're really interested in, like Wade Watts in Ready Player One by Earnest Cline, who is one of millions of witnesses to the hunt for Halliday's Egg. Because as much as the story is actually about Wade himself, it's the character of Halliday and his crazy egg hunt that I keep reading for.
So next time you're thinking about the metastructure of your story, ask yourself if the most interesting character is your MC, or if your MC is a witness to your most interesting character. It'll point you in directions you might never have gone otherwise.