My favorite is between Espeon and Lucario. I have a soft spot for gen 2 as I played it a lot as a kid and I also love Espeon's design. As for Lucario, the Mystery of Mew movie I watched as a kid made me bawl and I've had love for Lucario ever since lol.
Leavanny by far- I have a soft spot for bugs, an even softer spot for awesome bugs, and the softest spot for motherly mantises that can also easily sweep opponents. Its design is also super easy to draw which makes it all the better
Eevee and Latias. Eevee because Pokemon Mystery dungeon (even if eevee was probably one of the worse partners just because of run away) and Latias because that pokemon movie killed my feelings almost as hard as the final episode of Star Trek: Voyager.