My favorite books ever are Harry Potter.
As far as adult romance goes, I like Nicholas Sparks. My favorite book by him is "The Guardian," probably because it has a good suspense/mystery subplot, so it's not all romance (his books that are all romance tend to get boring for me), and it has good characters.
My favorite young adult author is Lurlene McDaniel... she writes romance, but it's the sappy, tearjerkery type (my favorite! LOL). Favorite book is "Don't Die, My Love."
My other favorite young adult author is Elizabeth Chandler. She also writes romance, but with a supernatural/suspense/mystery twist. She has a series of books called "Dark Secrets," but my absolute favorite by her (one of my favorite books of all time) is a trilogy called "Kissed by An Angel." I have the Collector's Edition, which is all three books in one, so I tend to think of it as one book, but technically it's a trilogy. It's soooo good. It's technically young adult; the main characters are in high school, but I can read it now, as an adult, and enjoy it as much as I did when I was in 8th grade and read it for the first time.