There are many pokemon known for giving affection to their trainers and to other pokemon. Jumpfluff are known for their use of sweet scent and attract, while Gardevoir need no attacks to attract more affection than they often can handle. And yet of all the pokemon in the world, the most affectionate of them all is not made of fluff or skin, but of sludge, for the most affectionate pokemon is Muk.
It is to their utmost discontent that a Muk trying to get a hug is like a Jigglypuff fancying itself a pop music idol. Their bodies are greasy to the touch, prone to sucking in hands, and prolonged exposure can result in illness; and perhaps worse, if a Muk will endure this, they still have no arms with which to hug their trainers back. It is far more common for them to use Frustration than Return in combat.
This is also a source of sorrow for trainers, for if a trainer endures their Muk's affections or even enjoys them, they will find themselves in possession of an extraordinarily powerful pokemon. Most Muk are fairly unremarkable in battle, but this is because their true potential lies unutilized; Muk, like magical girls and Birdramon, are fueled by the power of love.