I recently stumbled upon another book list - this one is someone's 10 Most Overrated Novels list. I pretty much agree with the list with the glaring exception of 100 Years of Solitude. (see my comments which are number 666).
The Modern Library has two lists - you can see them side by side - one is The Board's List and the other The Readers' List. There is a rival list from another publisher here.
The lists are fun, I think, since writing, like art, is so subjective. No two people would ever have the same list. You could measure popularity - most purchased, most checked out of a library. You could take a poll. You could see which ones are used most in classrooms. The possibilities are endless.
As far as my favorite books, well, I suppose that would change according the type of day I was having. I would say Little Women and Charlotte's Web were among the most important when I was a girl. I loved Jane Eyre, Uncle Tom's Cabin and A Tale of Two Cities, all of which I read as an adult. For contemporary fiction I'd say Bel Canto, Beloved and The English Patient. Non-fiction is hard to rate since it really depends on how interested you are in the subject, but I'd say that Women Who Run With the Wolves came to me at an important time in my life and has influenced me since.
I love to read biography, memoir and travel narratives for fun, but really good fiction can affect us more deeply, I think (and poetry too, for those who understand it).