He is not American. I know Hawking, the guy you mean. I asked for an American because I would like to interview someone like this one day.
http://nobelprize.org/nobel_prizes/physics/laureates/2006/ John C. Mather and George F. Smoot "for their discovery of the blackbody form and anisotropy of the cosmic microwave background radiation"
I can't remember his name but what about the black astronomer/physicist that was part of the panel to demote Pluto from planet status? I've seen him on the Daily Show and a History Channel show. Let me see if I can find his name. If he were still alive though, I'd definitely have to say Carl Sagan as the most reknowned physicist.
I would think Dr. Michio Kaku would be near the top of the list. He has appeared on TV extensively, where he has demonstrated a gift of communication with both laymen and professionals alike. He is a co-founder of string field theory and a protege of Edward Teller. Here is a link to his bio on wikipedia: http://en.wikipedia.org/wiki/Michio_Kaku
kaku was the co-creator of superstring theory, superstring theory brokedown when it split into 5 competing theories. Witten merged the 5 theories into a single theory by demonstating that the formulae were 5 perspectives of the same universe not 5 different theories. It is generally believed witten or widden is way ahead of the rest.
Feynmann died in 1988. Freeman Dyson is still around, though, and Kaku is a good science populist; but perhaps Alan Guth might be the pre-eminent cosmologist at this time. http://en.wikipedia.org/wiki/Alan_Guth Dyson was born in Britain, of course...