Underrated Science Fiction Writers at Meme Therapy

Meme Therapy asked a selection of people (including me) about underrated Science Fiction writers.

I found this a hard question! When is underrated just unknown? So I mentioned Christopher Priest because I'm surprised at the number of people who have never read naything by him, but in the UK he's critically acclaimed so how can he be underrated?

Good question, made me think. Good answers too from Elizabeth Bear, Karl Schroeder, David Moles and Shaun C. Green...

UPDATE
...and of course Jose!