I almost exclusively read fiction. I have a bit of a short attention span and need a story that will capture my imagination. I will read some non-fiction if I'm on a craze about a particular subject (e.g. music, religion), and I LOVE diaries and letter collections. But the bulk of my library is novels and poetry.
As for the best works I have ever read, for fiction it is Gaston Leroux's "The Phantom of the Opera", and for non-fiction it is "The Diary of Anne Frank". - "Out of Africa" by Isak Dinesen is a close second (there are too many close seconds on fiction for me to list!)