Cynthia in Champions Tournament grinds my gears. Her team is just a huge pain, especially Garchomp and Roserade.
Also most of the DRAGON trainers, DRAGON leaders and DRAGON elite four members as well as the DRAGON champion.
USE A GYARADOS
IT IS /NOT/ A DRAGON
But it certainly looks like one.
Critical hits grind my gears. They always happen at the most annoying times.