6. Groot & The Hulk
This would be great because of the deep, philosophical conversations the two could have. I jest, of course. This would be simply hilarious due to the limited vocabulary that the pair possess. I am picturing a fight scene with a moment of hilarity reminiscent of the Thor and Hulk team-up, after which it went quiet for a moment before Hulk punched Thor nonchalantly out of shot. In the same vein, I envisage the two brutes mentioned here about to fight a group of bad guys with Groot exclaiming "I am Groot", Hulk saying "Hulk smash" and the two proceeding to deliver a serious beatdown on their unfortunate foes.