-Sandslash ("Slasher")

-Donphan ("Phanphare")

-Torterra ("Sprout")

-Garchomp ("Chomper", "Takea")

-Nidoking ("Arthur")

-Golurk ("Ralph")

I think Ground is a smart type to have on your team, what with it being the only weakness of pure Electric Pokemon. Not only that, it has a lot of other useful advantages, and a lot of Ground Pokemon are just plain cool. How cool, you ask? The Nidoking and Golurk listed above were given the task of taking down Dragon Pokemon (Avalanche and Ice Punch, respectively), and they both excelled in that task. Also, both Garchomps were capable of demolishing Flint's team by spamming Earthquakes before the enemy could even take their first turn. I love them all so much...

Well, going by rarity of types, Fire and Poison are tied. Fire will be next, Poison will be after that.