My primary science hero is David Baltimore. What we know as a result of his intellectual endeavours is startling. He was always an exceptional student, but interestingly, during his undergrad, he lived in the shadow of an upperclassman by the name of Howard Temin (see below). He went on to grad school at Rockefeller, where he earned the knickname "the little emperor" from the esteemed facutly, as he never took shit from nobody; even Nobel laureates on the faculty would find themselves in over their heads when they attempted to criticize Baltimore's work.
He eventually went on to discover reverse transcriptase, which overthrew the "central dogma" (and for which he shared the 1975 Nobel Prize for Physiology or Medicine (at the age of 37!!!) with the aforementioned Temin, who independently proved the existence of reverse transcription). He also discovered the biochemical basis of VDJ recombination and NF-kB, among many other great accomlishments. He was also a pioneer in the development viral vectors for gene therapy. He continues to consistently produce top-notch science.
He was the founding director of the Whitehead Institute (interesting aside: while at MIT, he was one of three Nobel laureates in the biology department who were collectively referred to as the Good (Phil Sharp, who is a rediculously-kind man), the Bad (Baltimore, who was considered arrogant) and the Ugly (Susumu Tonegawa, who is supposedly a remarkbly unpleasant man). He has also served as the president of both Rockefeller University and Caltech. Additionally, he was an important influence on regulatory policies on recombinant DNA and has been a huge influence in HIV funding in the US.
He is well known for the "Imanishi-Kari affair", in which a collaborator was accused of falsifying data in a Cell paper. He stood by his collaborator, insisted that any abuse or allegations being thrown at her be thrown at him as well, and refused to retract the paper. A long, arduous, and well-publicized misconduct hearing led to his dismissal from the presidency of Rockefeller and polarization of the science world. His one-time friends, such as Wally Gilber turned on him. Mark Ptashne (presumably seeing trouble on the horizon from one of his main competitors) took the "whistle-blower" into his lab. However, in the end, Baltimore was vindicated, the results were proven true, and the "whislte-blower" was discredited. Baltimore moved-on with his life and became the president of the esteemed Caltech.
Finally, the number of incredible scientists that have trained with Baltimore is astonishing: some notables include Inder Verma (Salk), Gary Nabel (NIH), Richard Mulligan (Harvard), Sankar Ghosh (Yale), Steve Smale (HHMI/UCLA), Steve Goff (HHMI/Columbia), Frederick Alt (HHMI/Harvard), Victor Ambros (Dartmouth), and David Knipe (Harvard) among many, many more.
That's why David Baltimore is one of my science heroes.