I started making up my post for my Top Ten minor characters, only I came to a halt after a while because I found myself wanting to repeat a lot of books with my favorite minor characters. Harry Potter, The Hunger Games, Pride and Prejudice, these were pretty much the only books that my favorite minor characters are in. Perhaps not coincidentally, these books are also a few of my all-time favorites.
It's pretty easy for writers to flesh out the few main people we spend the book with. But for J.K. Rowling to have our hearts torn out at Dobby's death, or Suzanne Collins to have our sympathy reached for Finnick when we hear his past, or Jane Austen to have us anxiously waiting for Mr. Bennet's next humorous line--this is real talent, and what separates a good book from a great one. When every character, no matter how small, is as meaningful and alive as the protagonist and their close sidekicks, mentors, enemies, and love interests, I think that is a huge sign that this book will be memorable.
I wish I had a formula for making a minor character memorable and beloved, but I'm still searching for it myself. One thing I do think about it, though, is that stereotypes for meaningful minor characters (not just the old man on the bus without a name) is not the way to go. Make them as fresh and interesting as the main characters, with something just as unique about them as the protagonist.