Just in case anyone is interested, Humble Bundle has a Tad Williams bundle containing 21 ebooks for $18. The offer ends in about 1.5 days.

I've read his Memory, Sorrow, and Thorn trilogy. Epic dark fantasy, great characters and worldbuilding. The plot is good too, but the pacing goes off rail sometimes.

I really liked the "Otherland" series. If you are a fan of metafiction, you'll love it. Fundamentally it's sci-fi, but written in the style and cadence of fantasy. Without giving the plot away, the 4 book series takes you through historical events, imagined fantasy settings and through other works of fiction. There are elements that are perhaps a bit overwritten, but all in all it has a special place on my shelf.

The shadow march series is straight ahead fantasy, more cohesively (though maybe a bit less colorfully) written. Shadow march and shadow play were excellent. My attention waned a bit in the following two. Perhaps the same pacing issues the other commenter mentioned.

War of the flowers is a one off storyline that I remember enjoying, but it's tone was a bit more young adult if I remember right.

I can very much second the “Otherland” series. I’m not much of a fantasy fan (and can’t manage to sink my teeth into Williams’ other series because of that), but I found Otherland to be a very enjoyable and memorable series. Definitely worth it just for Otherland.