I would consider a philosopher; any person who introduces, or expands on an nonfactual idea or opinion in such a way as to provoke further thought. If a persons "philosophy" leads me to consider the validity of my own beliefs, than they, by my definition, are a philosopher.
As for who is a good one or not; I would tend to rank philosopher's roughly in order of how much thought their respective philosophies provoke.
As for Ayn Rand (who, by the way, is the subject of the aforementioned tangent); she very definitely provoked a great deal of thought.
Quote:About the Author: Ayn Rand (1905-1982) Novelist and philosopher Ayn rand, author of The Fountainhead, Atlas Shrugged and many other fiction and non-fiction works, is the originator of the philosophy of Objectivism. More than 20,000,000 copies of her books have been sold. In 1991 in a joint survey by the Library of Congress and the Book-of-the-Month Club, Atlas Shrugged ranked second (to the Bible) on a list of "books that made a difference" in peoples' lives. In 1998 a documentary film "Ayn Rand: A Sense of Life" was nominated for an Academy Award, and in the Random House/Modern Library Readers Poll, all four of Ayn Rand's novels were voted in the top ten novels of the 20th Century.