I feel like I have three favorites for different reasons.
Wooper, because its cuteness and silliness is endearing. Someone very dear to me ended up developing a whole personality for a Wooper I possessed, and he became very precious to me for all of the shenanigans and crazy headcanons we came up for him. Not to mention, it's got great colors, and Quagsire and Clodsire are both very huggable evolutions. I feel like Wooper is who I would choose as my first Pokémon if I were to venture out in the world as I am today.
Bulbasaur, because he was my first starter, and he was my favorite of Ash's Pokémon in the anime. My friend and I used to pretend to be trainers when we were kids, and recalling the memories of those adventures warms my heart. Nostalgia might make him a favorite, but I can't help it.
If I were to ignore my biases and just pick a favorite based on my personal tastes, it would be Swablu/Altaria. Birbs are my thing, and they're based off of the mythological bluebird of happiness, which I think is wonderful and very true to its design. It would be impossible to feel sad hugging one of those guys. I also love its typing and wish that mega evolutions were still a thing, because Fairy/Dragon is amazing and we need more.