For me it's a tie between Brionne and Grovyle I think Brionne looks super cute, and I actually prefer it's design over Primarina's. Really love the colour palette GF gave Brionne as well. Grovyle I've always been a fan of, and it kinda makes me sad that it evolves into Sceptile, which on the other hand I don't really like. I really love Grovyle's design, especially the long leaf on its head. Honerable Mentions: Torracat, Servine, Quilava, and Combusken.
I really like Grovyle due to his character in Pokémon Mystery Dungeon: Explorers. Aside from that, I also really like Dewott, Ivysaur, and Charmeleon.
My top two favorite middle stage starters are Charmelon and Grovlye cause they evolve into my two favorite last stage starters. Maybe a little bit biased in that decision.
Quilava, Dewott, and Servine are my personal favorites. I also like Charmeleon, Grovyle, Braixen, and Brionne.
Grovyle is the best, but I also like Quilava, Bayleef, Braixen, and Brionne more than their final forms. Then again, that isn't saying much for Bayleef.
My favorites are Ivysaur and Bayleef. Grovyle's pretty good too, I think I'd still count it as a favorite, but it's tail leaves are a little odd to me. Other than those three I don't really like the middle evolutions. Edit: I actually like all of the Johto starters at every stage.
I would probably have to go with Braixen as well, mainly due to its design and battle animations. I like them a lot more than Delphox's.
Braixen is my fave overall so far, it's just so cute! Notice how looks are always in the explanation for my favorite Pokémon In other generations my faves are Wartortle, Quilava, Grovyle, Prinplup, Dewott, and Torracat.
My favorites would probably have to be the Hoenn starters, Grovyle, Combusken, and Marshtomp. It might just be nostalgia, but something about their designs just appeals to me. I've also always loved Quilava.