If we're all doing this collaboratively, I regularly get OHKOs with Skuntank in 4th gen OU (and got one against Technical Machine recently). If you count VGC, I've also got KOs with Amoonguss, which might be a little hard to get otherwise. I could probably get one with Cottonee if necessary too, although I'm taking a break from server play for a while.
Pokémon spinoffs: Ranger You freak out when you hear a Clamperl; Mystery Dungeon You freak out when you see a Ledian.
<Sunflora> So let me get this straight: you're sending me into a dungeon full of lava, to capture a Haunter which is part poison type?