Asimov and Clarke and Heinlein were just the three who happened to be the most published in the Golden Age of science fiction. There were many others, some actually better story tellers, who just didn't get the favor of the publishers. Regardless, to comprehend and appreciate their tales and styles, one needs to read from a perspective of the era.
Certainly by the 60s, there were other authors coming about who were more appropriate for the time and sense of things. By the late 70s and on through the 80s (even holding through the 90s), there were several new authors who assumed the top ranks, and this happened again in the 90s with authors like Peter F. Hamilton and Iain Banks (Banks is actually a cross-genre writer, who was first published in the mid-80s, but whose SF came out and matured in the 90s), yet still there were many (indeed, many of these, female writers) who filled the background, not necessarily less capable, just not as popular.
(And though this decade there has been a new generation of writers, under 40, who may offer a lot. I don't know because I haven't really read any of them.)
However, it could be argued that some of these top writers did epitomize SF. Some of them are still prolific. Greg Bear is one of them.
Simmons is beyond SF, as his literary bent is somewhat classical in nature, not just in the particular style he approaches in a novel, but in how he portrays characters and themes.