I come up with Stephen Hawking and Jane Goodall at the top, with James D. Watson in the next tier down, maybe with Norman Borlaug (who has become famous for not being famous). Noam Chomsky is famous, and was a great scientist in his day, but his day was 1958. Lots of folks who had credible claim to being the world's most famous scientist -- e.g., Gould, Sagan, Feynman -- tended to drop dead at an early age. I think the last of the atomic bomb physicists are now dead (Hans Bethe died recently).
Who are your suggestions?
Of course, to be famous, it helps to be a public personality. Shy personalities like William D. Hamilton tend to be obscure, even though Hamilton's work was essential to better known figures like Richard Dawkins and Edward O. Wilson. So, a second question: Who is the greatest living scientist?