Even though I am a Seattlite, I would still say L.A. because it is the most "central" of the West Coast.
Dallas really shouldn't be on the list, don't make me draw on a map like a kid just to show you where Dallas is (hint, if you draw a line right down the middle of the US you will be very close to Dallas on the South part of the line.... and we already have 3 centrally located servers...
Really it's kinda moot though, the person setting up the server would decide where it should go.