Least Favorite Foes

We all have them; those Pokèmon that ruin everything. Just as you're climbing the Ranked ladders, or the Battle Tower, or while you're playing through the main game in general, this Pokèmon shows up and throws all your hard work down the toilet.
So, who are your least favorite foes? Me personally... I would say WALREIN. Every single Walrein I've ever come across (during the Battle Tower, Battle Tree and VS mode, anyway) is running the same set of Chesto/Stockpile/Rest, along with usually Snore and Frost Breath. Combined with its immense bulk... Yeah, I actually spent WEEKS looking for the perfect Pokèmon to counter it before settling on Tapu Koko. It's in my team mostly for Walrein. My second worst matchup is with Mudsdale (ugh, why must its ability raise its defense EVERY time it's hit?) If I'm specifically playing Gen 1, then my bane is Alakazam (which, along with the aforementioned tanks, is among my LEAST favorite Pokèmon of all time.

That stupid Cresselia with Toxic, Double Team and Moonlight is up there as one of the most annoying foes faced in a battle facility. That thing has ended many a streak in the Battle Tree because as soon as it gets going, it's really hard to stop, unless you're willing to lose a moveslot for Taunt. The only other recourse is trying to Haze away the stat boosts or use Dragon Tail, Whirlwind or similar moves to remove it from play, but that won't do anything if it's the last mon.

A lot of gym leader's aces get on my nerves. Especially since I tend to have pokemon with ineffective types or movesets against them like Whitney's Miltank or Morty's Gengar

For some reason I don't really like when swimmers encounter me while I'm surfing ): idk why

I mean, if we're taking the GameBoy TCG video game into account, then Chansey is on there, too. Stupid Scoop-Up combo...

Stunfisk at first really annoyed me because I couldn't figure out a reasonable Pokemon to take it out. However, now I would have to say Wally's Kingdra peeved me, whenever I had to go up to challenge it and Elektross because of its Levitate ability.

In B&W, Dragonite is the one that really threw me down the stairs of the Black Tower. I never had a real counter for it and it's way too bulky to just scrap with, so I had to make heavy use of my Toxic TM.