Dragonite is not as good for training as it first might seem. Dragonite's strength comes from its insanely high stats (hence the ridiculous CP). This is why it's able to beat everything else. However, if you reduce its level to be half the CP of the defender, Dragonite is suddenly not so great anymore. Basically, best training Pokémon are strong Pokémon that have relatively low CP, like Wigglytuff. Dragonite is the complete opposite and in my opinion is not that great of a prestiger. I thought this too first when I caught a 600 CP Snorlax, but when I trained with it I realized that it had close to no damage and was a horrible prestiger.
So naturally high CP Pokémon are great for attacking and defending, but relatively weak for prestiging.
