Ditto

When a Pokémon breeds with Ditto, the egg is always the partner's species, so no Ditto ever hatches from one. And since it pairs with almost anything, a Ditto from a game in another language is the usual way to run the Masuda method for more shinies.

Ditto is the only species in its group, and it breeds with 873 of the 1025 species: all of them except the 151 in the Undiscovered group and another Ditto. For the 39 genderless species that can lay eggs it is the only possible partner, and it spares single-gender species from finding one of the opposite gender.