It depends on what the scientist does. Nowadays I would say that what most scientists need is a computer, which was not the case 25 or 30 years ago. Scientists also need books, lots of them and databases where they can find useful information for their research. Some scientists need especial equipment for measurements or analysis, and they also need tools, different tools to perform the kind of work/research that they are dedicated to do.

Scientists need also time, passion, understanding, and ethics. Although those are not tangible tools or instruments, they are fundamental tools to do research and generate useful results for the advancement of Science.