In general, I'd expect sentences that convey information about the scientific process to be much superior to sentences that contain merely some clever concoction of facts. Analogous to teaching a man to fish versus just giving him some fish he would have caught anyways given the right method. The only exception would be information that is extremely unlikely to be experimentally discovered* (yet the previous generation stumbled upon) but the expected value of which would outweigh some optimization of the scientific method. (*Before a cataclysm)
Come to think of it, the most such important piece of information (and also keeping with the hypothetical) would be information about the cataclysm (only applies for anthropogenic cataclysms). The reason is that ceteris paribus, a similar pattern (which we would even help along with our usual sentences) could well lead to a similar outcome (another catacylsm). Gotta break the circle.
Alternatively, what about "ALL THESE WORLDS ARE YOURS EXCEPT EUROPA ATTEMPT NO LANDING THERE". Imparts some important cultural information. Hope the poor blokes don't take it too literal and implement a naval embargo of Europe.
So apparently Richard Feynman once said:
I could be missing something, but this strikes me as a terrible answer.
When was the atomic hypothesis confirmed? If I recall correctly, it was only when chemists started noticing that the outputs of chemical reactions tended to factorize a certain way, which is to say that it took millennia after Democritus to get the point where the atomic hypothesis started making clearly relevant experimental predictions.
How about, "Stop trying to sound wise and come up with theories that make precise predictions about things you can measure in numbers."
I noticed this on Marginal Revolution, so I shall also state my candidate for the one most important sentence about macroeconomics: "You can't eat gold, so figure out how the heck money is relevant to making countries actually produce more or less food." This is a pretty large advance on how kings used to think before economics. I mean, Scott Sumner is usually pretty savvy (so is Richard Feynman btw) but his instruction to try to understand money is likely to fall on deaf ears, if it's just that one sentence. Think about money? Everyone wants more money! Yay, money! Let's build more gold mines! And "In the short run, governments are not households"? Really, Prof. Cowen, that's what you'd pass on to the next generation as they climb up from the radioactive soil?
*Cough.* Okay, I'm done. Does anyone want to take their own shot at doing better than Feynman did for their own discipline?