I'd choose one of these two, since they both completely qualify as underrated:<p>A) "Psychohistorical Crisis" by Donald Kingsbury, which is an unauthorized sequel set in the world of Asimov's Foundation Series.<p>B) "Venus Equilateral" by George O. Smith, which is an old, rather cheesy sci-fi novel from the 1940s, which is basically a bunch of engineers on a communications space station fighting lawyers, bureaucracy, and space pirates through inventing stuff.
I enjoy Adam Roberts, and he doesn't tend to get much mention on HN. I particularly enjoyed <i>Salt</i> and <i>New Model Army</i>.<p>I also enjoy <i>Allen Steele</i>, and again he doesn't get much mention on HN. I really enjoyed the <i>Coyete</i> series, although many people get stuck on the lack of science in them.<p>I don't know about underrated, but I also loved reading Interzone when I was younger, and the anthologies are great if you can get hold of them.