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

Don't make this poor soul search all 1,025 pokemon on there

I'd suppose Guardevoir or Vaporeon

The older games still hold up at least. I’ve been playing through Emerald again and it’s been a lot of fun.

There’s also quite a few good ROM hacks out there too.

I still think Gold/Silver/Crystal was the actual peak in terms of world design/mon design/storyline/characters, but I'm 100% willing to admit that could be my own particular age bracket of nostalgia talking.

I do like some of the additional/revised gameplay mechanics from later games... team battles, weather, held items, that kind of stuff makes the games meaningfully more complex, as well as more detail and structure given to how breeding and heritability actually work.

But a lot of the more... closer to current day stuff bssically just seems entirely like a gimmick that isn't very well systemically thought out.