Charmeleon, Wartortle, Vulpix, Machop, Mime. Jr, and Slowpoke.
Charmeleon and Wartortle are honestly really hard to explain, but I just really love them. I always have, and probably always will. Something about these middle stage evolutions has always caught me as "lovable", idk. Maybe it's because I really like turtles, for the most part, in Wartortle's case... Charmeleon is just cool looking. I guess it's just a great and not awkward stage between Charmander & Charizard. Also, Zippo when it was a Charmeleon was really cool.
I suppose Machop is out of nostalgia, for the most part, though. There's a weird story about why it's among my favorites.
And Vulpix is just... so cool. I love the lore behind it, and also, I love red. Vulpix is red, and a fox. Foxes are cool. The idea that it starts out with one tail and then it splits into six tails is just an amazing concept for a Pokemon. I also have fond memories of catching a shiny Vulpix... and that makes me happy, so Vulpix is pretty important to me.
For Mime Jr. I don't know, honestly, I think it's because of James having one in the anime, and it being so funny. Also, I find the way it evolves to be really cool and interesting.
And Slowpoke is just... adorable. It was a favorite Pokemon that came out of absolutely nowhere. I actually used both a Slowbro and a Slowking before in battle, and over the years, the Pokemon has just grown on me from that experience. Also, it's pink, I love pink.