Iron. I would rather have medieval infantry than horsemen, and I would rather try to win with medieval infantry than survive to a later age and win with infantry, for example. Iron also allows for factories. Horses are only good for horsemen without iron or saltpeter. Saltpeter would only provide musketmen. Coal is useless without iron. Oil by itself only provides ships. Aluminum is pretty useless by itself. Uranium by itself only provides nuclear submarines and the Manhattan Project. If I were playing as Egypt, Iroquois, or Hittites, horses would probably be more important.