Lucario. Even mentioned on their Wikipedia page:

Patricia Hernandez, in an examination of the furry fandom, stated Lucario was the most popular Pokémon for the subset dedicated to the franchise’s characters.[45] Meanwhile, the Pokémon has also been cited as one of the most frequently utilized in erotic works by the fandom and furry pornography,[46] with a June 2023 study of such content on Rule 34 websites, such as e621, Rule 34.xxx, and Sankaku Channel, noting a significantly higher volume of material compared to characters from most other franchises, and the highest of characters from the Pokémon franchise as a whole.

https://en.wikipedia.org/wiki/Lucario