Final color plus scale for me.
Under 1/350-1/400 scale, preshading is rather pointless--at a 12" viewing distance, you area scale 350-40' away. I might preshade at 1/60-1/72, but would really not bother until 1/48.
Final color, and which part of the hull, do seem to matter, though. For instance, on black hulls, I really like priming in silver or a very pale gray.
For gray hulls, I kind of like using a blue primer.
For superstructures, I will use white or radome tan for primer. For the most modern USN ships, Iv'e taken to using Sky Type S, as that sets a really good base for the pre-catylized paint in current USN use (and the "easter egg" look repainting gives).
For white hulls, pearlescent primer is the bomb. But, for Great White Fleet ships, red for the hull, and ocher/tan for the superstructure really seems to be the key.
For decks, I near universally use a blue primer.