the town to the South-East of Thaspus
Lepcis Magna. A city on Sicily, or Croton might be more centrally located. If you own all of Greece, Athens makes an excellent capital, though your Spanish conquests won't like that too much

I believe in the "Distance-to-Capital" thread above, there is a link with an interactive map that shows you the penalties to every city on the map based on the city you choose for your capital. After awhile it just becomes intuitive as to where to place it, but it's good to know what the actual numbers are