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!
Oddish is my absolute favorite. It really embodies the, "I'm just doing my best out here" kind of vibe that I really resonate with.
Otherwise, I think Flygon is a close second because of its excellent design.
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.