The average American would be hard pressed to name any famous writer at all. -- "Uhh...the guy who wrote Shakespeare?"--  Personally, when I was a kid, I knew who Wells, Verne, Bradbury were.  It was a long time before I heard of Heinlein, but I read mostly non-sf.  I certainly knew who Rod Serling was, but I don't think I would ever have mentioned him as a science fiction writer.  To me he was a guy on TV, and I knew he'd written REQUIEM FOR A HEAVYWEIGHT which, though I hadn't seen it, I'd heard so much about that I thought it must be something very Important and worth remembering. 

But it's true that, thanks to Twilight Zone, I soon knew who Richard Matheson and Charles Beaumont were.  And I bought a paperback copy of More than Human when I was very young, so I knew Theodore Sturgeon.

My son, who reads a LOT, knows Bradbury, has read Fahrenheit 451.  He knows Arthur C. Clarke,  because I've been pounding it into him for years.  But a year or so ago, he asked me for some science-fiction novels he should read.  I suggested, among others, Stranger in a Strange Land, Left Hand of Darkness, and Ender's Game.  At that time, it was the first he'd ever heard of Heinlein, LeGuin, or Card. 

I just think there's a big difference between the average American, the average literate American, and the average sf-reading American.

It's really hard to quantify such nebulous things (Most Popular?  Most Famous?  Sez who?).  But my feeling is, that, during Bill's time-frame of late 40s, early 50s, Ray Bradbury would have been the most famous LIVING sf writer.  But that's just a feeling.