I like what I have read of Kaku, but I've never heard him broadcast.

I'm possibly biassed, but I think Martin Rees has it all: good general scientific knowledge and a remarkable gift for simple explanations of complex scientific issues. However, he is probably far too modest to try to pass himself off as "the new Sagan" (at which juncture I probably ought to confess that I think Sagan was over-rated - very, very good, but still over-rated -- and Rees is very much under-rated).

--

Stephen