I'd say Arcanine, judged upon his moveset, and even and high stats.
Nature: +Speed -Sp. Defense. (Sorry don't know what that nature is)
Item: Heat Rock
EVs: 252 Speed, 252 Attack, 4 Sp. Attack
Flare Blitz: Stab, good physical move.
Outrage: Takes care of dragon types, which I see a lot of. Pyhsical move.
Solar Beam: Takes care of all of your weaknesses.
Sunny Day: Increases Flare Blitz power by 50%, also makes Solar Beam instant.
Fire's only weaknesses are Water, Rock, and Ground, and it just happens to be, that grass destroys all of these types. So, you send out, Arcanine. Use Sunny Day. Now Flare Blitz's power is incredibly high and Solar Beam is instant. Outrage is there to deal with dragon types. With your Heat Rock there, your Sunny Day will last longer you'll hopefully be able to take out a good chunk of their team with Arcanine. With his now high Speed stat you'll be able to go before any Politoed, setting up your sunny day, lowering their water attacks, and making Solar Beam instant. Your only problem will be fire types, and I'm pretty sure your outrage will be a 2HKO.