In the games, Pokémon Trainer is used as the Trainer class for certain storyline characters. These opponents tend to be the most powerful non-arena Trainers the player will face, notably including rivals, Red from Pokémon Gold and Silver, Crystal, and HeartGold and SoulSilver, Steven from Pokémon Emerald, the Stat Trainers from Pokémon Platinum, Cynthia from Generation V, and Rood and Alder from Pokémon Black 2 and White 2.
As for Generation 6, Emma (or Essentia) and AZ have the class of Pokemon Trainer, giving them the strongest AI (since each trainer class has set AI). Otherwise, Diantha the Champion has the strongest AI.
Source
