Probably on the 17th floor to put the generators closer to the loads and reduce transmission losses. Also reduces street level air and noise emissions (and avoids street level air intake)

With fuel storage on the same floor for a reasonable runtime, larger fuel storage in the basement and a pump (or two) to move fuel, you're well prepared for lots of things. As long as the fuel throughput on the stairs is better than the fuel use rate, then you're ok for as long as you've got people to move it. Which is ok for exceptional incidents.