I'm going to go with Diantha too. Being able to direct Mega Gardevoir without voice commands takes great skill and an unbreakable bond with your pokemon, not to mention being a huge advantage in battle (nothing worse than telling your opponent the move you will use before you use it). No other pokemon trainer that I know of in the anime can do this on a regular basis.
Of course, I could always cheat here and say Mewtwo, who communicated telepathically with the pokemon under its command, similar to Diantha. You can argue whether or not Mewtwo counts as a trainer though.