I personally recommend Zealot, because it allows you to keep the planet instead of just destroying the cities.
NOTE: If you want to do it this method, you need to be either a Zealot or a Scientist because of their unique Fanatical Frenzy (Zealot) or Gravitation Wave (Scientist) abilities which break the galactic code. Allying the Grox is not an easy task, however, there are many things you can do to get on the Grox good side without needing to grind for Diplomat 5 or Conquerer 5 to get Embassy or Planet Buster.
