Large Black Slug (Deroceras reticulatum)
This Large Black Slug also occurs in various other forms including brown and a chestnut versions. Although it will apparently eat anything it is not a species that does much damage in the garden, that honour falls to the smaller Netted Slug.
I guess the poor slug is not going to be popular with anyone as it has little going for it. However, Hedgehogs, Frogs and Grass Snakes are all very partial to a tasty slug so someone loves them.