I know Sylveon is my pfp, but Espeon is my actual favorite. I started with the originals, so initially Eevee was something you only got one of if you didn’t trade. That made it special, and of course it was cute as a button, so I always tried to get one regardless of the game. Needless to say, I was super excited to find out you could catch wild Eevee in Gold/Silver. So I caught one and was determined to try to raise its friendship to get one of the new evolutions. It just so happened to happen during the day, so I got Espeon. The friendship mechanic made an already special pokemon that much better. It helps that it has a great design and a cool type, too!
Slowbro is my favorite. Slowbro is resilience through adversity. Slowbro evolved by drawing power from misfortune. Slowbro may be slow but Slowbro pulls through. Slowbro is a lesson in life. Special mention to Mega Slowbro.