While Metagross is indeed always a good choice it does in this case give you a x3 weakness to groudn type attacks, which depending on your own prefference may or may not be a problem. alternatively you could look at Rotom for a good electric/ghost/water choice in your team and it eleminates the x3 ground weakness.
Alternatively there are several other viable options: porygon-z, Lucario, whatever mystery egg pokemon you get, Volcarona or Eelektross. it all depends on what you're looking for in a sixth team member.