It would be so much easier to pick a universally liked science fiction book (Ender's Game by a mile).

For fantasy, it's a little more difficult. "Universally liked" is very difficult, it would be better to clarify if we're going for the book/books that are liked by the most people, even if they're actively disliked by a substantial number of people. [Which would disqualify Martin, Jordan, and Tolkien because I hate to say it but it's too dry] Are we going for a book that's at least ok for the majority, or that's the most loved by the most people, no matter how much its hated by others? If you go with this definition, you could even make a case for Atlas Shrugged. [Ender's Game wins the sci-fi slot by any definition you can go with- practically all sci-fi readers have read and like Ender's Game, and many love it]

I am choosing to go with books that are (or could be) well liked (not just acceptable) by the vast majority of fantasy readers and are unlikely to be hated by too many people.

Most likely, the answer is Robin Hobb. It's fantasy that's well written but easy to get into and entertaining, it's not too different/weird while not being derivative and unoriginal. I don't know of many people who've tried Hobb and didn't like it and I believe it's pretty popular. I would also say C.S. Friedman- her fantasy is also very good and popular.

Guy Gavriel Kay is a possibility, but it's less adventure oriented so it's a little in the alternative category so I don't think he has as much mass appeal as Robin Hobb, C. S. Friedman, or Carol Berg. [I really really wanted to say Carol Berg just because I can't imagine someone not liking her fantasy but she's a fairly recent author and I don't know how many readers are aware of her as of yet.]