I think this kind of thread has been done before, but meh.
Sometimes a Pokemon or their moveset is so specifically constructed that it can be really frustrating to fight against, and as a result, really fun to use! Have you had any favorites?
My first example of this was the beloved Trapbat from GSC: a Crobat with Toxic/Fly/Confuse Ray/Mean Look. There's something really delightful about completely flustering and dismantling a Pokemon without actually inflicting much direct damage. Of course, it was boned if the opponent had a Thunderbolt or similar, but against unprepared enemies, it was massively effective and cheap.