I rotated my two highest Gyarados, Dragonite, Exeggutor, and one of my Vaporeon... so yeah, basically my highest level Pokémon.
There was a clever strategy that I heard about recently. Many people know about bulk evolving Pidgeys, Weedles, etc. for the experience. This guy told me he'd bulk evolve and use the evolutions to fight gyms, even if it takes six Pidgeottos to take out one Pokémon. He'd then transfer the six fainted Pidgeottos and use the next group of six to take out another Pokémon at the gym. This way he saves Potions and Revives.
Talk about getting value out of the experience fodder and managing resources...