Difficult/Challenging Authors

[Evil Agent]

I'm wondering who are some of the most difficult/challenging but rewarding fantasy authors you've read?

I ask this because I've just ordered a bunch of Gene Wolfe books (Wizard Knight and Book of the New Sun). It may be a while before I get around to reading them, but everything I've heard leads me to believe that reading Wolfe will be a challenge. This could be good, however, as I've lost interest in a lot of fantasy over the last few years and rarely enjoy any new authors.

For me, the most challenging/difficult but ultimately rewarding authors I've read are probably as follows:

Stephen R. Donaldson (Blew my mind when I was 19, after having mostly read Eddings & Goodkind).

Mervyn Peake (Some of the best prose I've read. Highly descriptive, vivid imagery).

Patricia McKillip (Riddlemaster was one of the most bizarre books I've read. Still not sure I understand what the heck happened).

I can't say any of them were my favourites, but they were all enjoyable in their own way.