I recently discovered N.M. Browne who writes stand-alone YA books which are brilliantly written and very clever - I loved both Shadow Web and The Spell Grinder's Apprentice.
Also, I recently read The Demon's Lexicon by Sarah Rees Brennan, which was also wonderful - and funny. It's the first of a trilogy (sorry, I know that's not what you wanted) but it would stand alone too.
Robin Mckinley is almost always excellent and, of course, absolutely everything by Diana Wynne Jones is amazing.
Also, I recently read The Demon's Lexicon by Sarah Rees Brennan, which was also wonderful - and funny. It's the first of a trilogy (sorry, I know that's not what you wanted) but it would stand alone too.
Robin Mckinley is almost always excellent and, of course, absolutely everything by Diana Wynne Jones is amazing.