It is not really possible to pick out only one principle, but here are three that would be at the top of my list. The first one would be recognised and accepted by almost every scientist. The second one might not be recognised by every scientist, but I think would be accepted by every scientist. The third one would definitely be disputed by many scientists, but I think that it correctly captures the practice of scientists, no matter what they claim to believe when they enter into philosophical discussions.

First, the Copernican principle - which states that human beings and the Earth are not at the centre of the universe. Indeed, there's no evidence that the universe has a centre. This should teach our species a certain humility that it sorely needs with respect to the ecosphere, and that certain classes and nations sorely need with respect to the rest of humanity.

Second, the principle of non-authority - which states that scientific truth is judged by observation and experiment, not by appeal to authority. The same is true of truth in general, in my opinion, and I think the application is obvious. One only needs to think of recent wars and their justification, and controversies in the USA over the evolution of species.

Third, the principle of materialism - which states that matter is primary over ideas, perceptions and spirit. Ideas, perceptions and spirit emerge from matter, and not vice versa. The application to human affairs is more subtle here. But ultimately, I think that the principle of non-authority rests upon the principle of materialism.

See Rafael Sorkin's website.