My favorite Pokemon is Delcatty! It's probably mostly because I played through the entirety of Pokemon Y with one, and it was the highest leveled Pokemon on my team.
Other than that, it's a cute cat and I usually just choose my favorite Pokemon based on appearance.
Sadly, it hasn't been obtainable since Gen 6 [Well, technically also in Brilliant Diamond/Shining Pearl, but I've had no luck].
