I asked my students at SDU about it. Among options that I gave them were equipment and facilities, employees, international mobility and a Big Name of the University. The latter might help in getting funded, right?
Their answers surprised me in a good way. My students value most working in a small, but good group, fairly well equipped.
No need in a big name of the University or extensive research mobility. The latter only if needed for their research.
There you go! I think PIs can learn from listening to young researchers:)