[AWoodside]

It's hard to pick just one person as scientific fields are so vast these days, but for physics I would have to say Nima Arkani-Hamed for several reasons.

He's in many ways a string theorist but is not nearly so dogmatic as the majority of people working in that field. He focuses on developing theories that are empirically testable and has a few that will be verified/falsified as soon as the LHC comes online. His work on the hierarchy problem has also gotten him an international reputation as one of the best emerging physicists, and he's set records in terms of citations and academic accolades (not that this necessarily means anything, but I think it's a good indicator) relative to his age.

In addition to being a brilliant theoretician he is hands down the best teacher I've ever had, and everyone else that has taken a course from him feels the same way. The physics community suffers a lot from poor teaching in my opinion and I don't think that the value of getting the next generation of physicists interested/excited about physics should be underestimated when looking at the long term future of the field.

Anyway, I'm a little biased, but that's my 2c.