Is it OK if I love a whole line? Because, the Rowlet line holds a special place in my heart. Rowlet is adorable, Dartrix is cool, and Decidueye simply awesome. (and their shinies! How beautiful!) Because of this, the grass owl's evolutionary line is one of the major things that turned my attitude towards Generation VII upside down, from a hatred to a love.
If I had to choose one Pokemon, however, I would choose Silvally. Its design is unique, and its backstory one of the most intriguing. I know it's just meant to be a weaker version of the Great Alpha Arceus, but to me its more than that.

Mega Lopunny because god damn those thighs it's really good in competitive, and is a staple on the teams I use. It's also really cute.

Oh boy, I have so many favorites that it's hard to choose just one! Mimikyu is my favorite non-legendary Pokemon because of it's adorable appearance and tragic backstory. But if I had to choose a favorite legendary, it's definitely Mewtwo because it's intelligent, powerful, majestic-looking, and is the only Pokemon so far to become a well-developed character with motivation and goals. :P

Pheromosa (aka UB-02 Beauty). It looks amazing, it's the third fastest Pokemon in existence, and it's pretty good in battle as well.

DITTO !!!