Some good points on Simmons. Harlan Ellison certainly would agree that Simmons is a great writer.
But the best?
Hard call.
I have really enjoyed introducing my daughter to Simmons' stuff, and she and I had the pleasure of meeting him in person some years ago. His visit of poetry and the debate of art vs. truth is fascinating.
Writers I always advise people to read would include Rudy Rucker and Vernor Vinge, although they are not so popular in sales. Their stories are compelling even as their styles are very different.
Gregory Benford's Galactic Center, I agree, is really good. Especially the earlier novels where Walmsey is a prominent character.
'THE best' is a tough target, imho.
Ed