Back in gen 6 I couldn't stand clefable because it gained the fairy type. Plus, is new typing allowed it to strengthen its gimmicky sets like the "Stored power + Minimize + comsic power + soft-boiled" with magic guard. That in particular was a nightmare for me to face on battle spot because of the timer and only three mons being allowed per battle.
I finally released my first Official music Video of my new single Lulla-bye:
I don't like facing Gothitelle very much. I dislike how PP stalling passive stuff with Shadow Tag is the whole reason to use it.
Last generation I hated facing Alomomola in RarelyUsed. It was extremely hard to wear down thanks to its huge defensive stats and pure Water typing, had Scald which, among other things, forced Swords Dance Virizion into running Lum Berry and thus caused it to be painfully weak before a boost, and was able to mitigate things like Registeel or Diancie having no reliable recovery with Wish which made it really hard to break through defensive teams. I always thought it should have been banned but I'm not sure if people have worked out how to deal with it in the years since sixth gen ended.
i have always hated fighting breloom. spore + huge attack stat and the ability to either abuse supbunch or technician SD. you need to be very careful with how you deal with it.
Thankfully, Breloom is significantly less threatening than it used to thanks to a pretty good number of mon that can absorb spore and either stall it out or kill it. Tapu Bulu and Mega Venu in particular come to mind.