Here's an answer I doubt anyone else would say: Lucario. It hits hard, and it's fast, which is something I do in battles a lot. And with low defenses, it takes me out fast. I despise fighting it, too, considering how much I adore them.
Ok I have to add to this- literally any stall Pokémon. Porygon2, Chansey, Blissey, SHUCKLE, etc. It’s a terrible playstyle and I don’t like it. I can’t stand stall teams. It’s the worst way to lose. I was on Pokémon Showdown last week and my team got stalled to death and I was furious.
There’s two trainers in Kalos and Unova that use Kadabra really early in the game. With it’s solid special Attack, it can easily sweep your team if you can’t outspeed it. Also Pokémon with sturdy are infuriating.
If I'm playing a Nuzlocke of any kind, Wobbuffet. Out of all Pokémon, they tend to scare me the most when one appears on the field. But if not a Nuzlocke... then I would say Frillish/Jellicent. They're incredibly annoying, especially if they have Recover.
*inhales* EVIOLITE CHANSEY *exhales* You're pretty much doomed if you have no physical attackers on your team going up against one.
In a casual run I don't have much of a problem with anything, but in competitive Landourus-T is the bane of my existence. If it didn't have Intimidate I would have no problems with it.