Fire/Fairy :as there no mons that share this type combo it gives a wider and more interesting options.
Ground / Flying:Both cover each others weaknesses(to and extent) and give some fun options to use.
Ice/Fire:just a very fun and interesting combo and to a good extent hard to.
Normal/Ghost:An interesting combo specially if you consider one is arguably the most common type in game after water and the other the rarest with also giving and interesting coverage to each others weaknesses .