I usually have a hard time with this question, but for Gen 6, it's Espurr, hands down. It's cute, creepy, and it's a psychic kitten. What's not to love? I also really like both Meowstic forms,
and their "differences, so that helps too. I just wish they were stronger, stat wise.
Honourable mentions go to: Boweon (I hated Sylveon at first, but after using one, my opinion took a 180),
Braixen (fox+witch=win), Xernas (fairy deer is best leggie), Goomy (who doesn't like it?), Liteo (LIONZ), and probably Flechling (robins are one of my fav birds).
Maybe missing one or two, but yeah cx