Dennis is being silly.
Asimov was one of the Big Three, with ACClarke and Heinlein - the shaped SF as we know it today, would be very, very different without them.
You've heard of the Three Laws of Robotics? He da man.
The Foundation series is one of the best known SF novel series ever, and for many years was at the top of everyones' lists, kind of a 'Citizen Kane' of SF.
Personally, I don't think he's a great prose writer by any means, but his big ideas sweep you along. And he won a stack of awards over the years.
Read his Wikipedia entry.