Scientists must have Integrity.
Because Science involves an ethical principle, the love of truth, its practice cannot be unmoored from principles of morality. A hypothesis can only be put to a fair test by a person who prefers knowing the truth even if it shows that their previous position was wrong (a corollary is that science becomes unreliable when there is political pressure to come to particular conclusions, such as the Lysenkoist biology mandated by Stalin, or the Deutschephysik of Nazi Germany).
This virtue is sometimes referred to as “objectivity”, but this word suggests a sort of dispassionate neutrality which is not actually characteristic of most real scientists—we actually tend to get rather excited about our work, or we wouldn't be doing it. A better term for this virtue is humility: when doing research the scientist must take the posture towards the universe of a learner, rather than a teacher. Unfortunately, some effective scientists are conceited and arrogant towards their peers, but when scientists take the same attitude towards Nature, they continue to defend ideas long after they become discredited, and become useless to Science.
It's also extremely common for working scientists to get mailings from laypersons who believe themselves to have revolutionized large areas of science, despite having imprecise, untested, and often meaningless ideas. This psychotic disconnect from reality is nearly always accompanied by severe egoism, showing by contrast the way that humility characterizes true science.
Of course, humility does not involve taking the view that all knowledge is unreliable and tentative, since this would actually inhibit the discovery of truth! (In modern times, revolutions in Science usually do not totally invalidate our previous understandings; instead the previous theories survive as approximations.) The proper attitude of a learner is: “Test everything; hold onto what is good” (1 Thessalonians 5:21).
A second virtue of science is honesty. Scientists must refrain from fudging results or misleading other scientists. Honesty requires noting the factors weighing against a conclusion as well as those weighing for it. They must also take precautions against bias, not in the sense of being unbiased (none of us are), but preventing that bias from contaminating their results. Hence the need for experimentalists to do proper error analysis, use control groups, double-blind tests, etc. Experiments that show the absence of an effect should be published as well as experiments that show the presence of an effect, even if such results are less likely to result in fame and respect.
I was going to include some juicy long excerpt from Feynman's famous commencement address on "Cargo Cult Science". But too much of it was relevant to what I'm saying! You should just go and read the whole thing.