DISCUSS any Pokemon you hate fighting against/ hate trainers who use them.
For me; Blissey trainers, Blissey teams, Blisseys I'vee had to endure through, evil Blissey movesets, etc. Personally, I've fought this one guy who uses Blissey with the move Healing Wish (or something like that). It KO's their Pokemon instantly, but fully heals the next Pokemon thrown out. Try having your team suffer through Tyranitar, then him switching to Blissey, that KOing a couple of yours then it KOs itself, to bring back Tyranitar AGAIN!