CHOICE SCARF WAILORD.
MY...SWEET...BABY....NEUROSCIENCE
Move set
Water spout
Brine
Ice beam
Earthquake or Self destruct or something.
If you don't know, Water spout has more power the more HP you have, with a maximum of 150.
Brine does the opposite, when you are lower on HP it increases in power maxing at 120.
Water spout at full HP 1HKOs many grass types. And because its base speed is 60, with the choice scarf, only the fastest of the fast are capable of outrunning him.
If you run into a particularly bulky grass type, ice beam.
The choice scarf means you're stuck in one attack until you switch out.
Wailord's incredible bulk and his resistance to 4 common types means he can switch in with a lot of options, anything damaging him 1x probably won't kill him.
He can even shrug off electric and grass attacks, except for powerful physical attacks from attack oriented grass types (I'm thinking of you leaf blade).
More recently
TOXIC ORB URSARING
Ability: Quick feet
Move set
Facade
Close Combat
Shadow Claw
Swords Dance
Quick feet raises his speed to 1.5x which with my EV trained Ursaring at level 50 gives him 150 speed, which is VERY quick.
Toxic orb means he gets a stab 140power attack.
Close combat gives him coverage against things that resist normal, and Shadow claw kills ghosts.
Swords dance is self explanatory, use it the first turn, then sweep when you get your poison.
It has a major weakness in a fast pokemon using a fighting move before it gets its speed boost, but besides that, it's a wicked sweeper.
Additionally, Mamoswine with a choice band and Snorlax, but I'm not going to go through all the trouble of what I did for Ursaring and Wailord.