Your choice is between the energetic Scorbunny, the sensitive Sobble, and… Grookey, which is a green monkey with a stick.
But which one is the best choice?
Scorbunny is a fire type who evolves into Cinderace. He learns a 120 power unique fire move which is sort of like Flare Blitz but without the recoil. He has moderately high attack (116) and good speed (119). His design is pretty cool. He has a Hidden Power called Libero, which is functionally the same as Greninja’s Protean ability. It changes his type to whatever move he’s using. If Libero was currently available (it’s not), Scorbunny would be far and away the best choice. Without Libero, Cinderace is a decent pokemon. Cinderace’s fire typing is strong against the first gym (grass), which doesn’t matter because it’s a very easy gym and you almost certainly have a strong bird pokemon to take it on, and weak against the second gym.
Sobble is a water type who evolves into Inteleon, which looks like it’s been pulled out of… Ben 10 or one of those other Gen Z shows. It learns a unique water move called Snipe Shot, which is a worse version of Scald, but it looks cool. Of the starters, Inteleon gets the most optimized stats for a sweeper, which is pretty much the only thing that matters for the base game. 125 Special Attack and 120 Speed. He’s less dependent on his unavailable Hidden Ability that the others, which increases critical hit damage. Inteleon’s typing is weak against the first gym, which as I mentioned, I don’t think matters too much, and is strong against the third gym, which also doesn’t really matter because by then you could have a Gyarados or another strong water type.
Grookey is a grass type pokemon that evolves into Rillaboom and I hate it. Grookey is boring, it’s middle evolution is just gross looking, and Rillaboom is super extra. It learns a unique grass move that’s pretty strong (80) and lowers speed. It has great attack (125), decent speed and good defensive stats. Its Hidden Ability, Grassy Surge, gives it a weather advantage, which is nice. I don’t think Rillaboom overall is a game breaking pokemon, just average. However, grass types are usually even lamer than this pokemon so if you’re looking for a grass type he might be your guy. He’s neutral against the first gym, strong against the second gym, and weak against the third gym.
To me it’s a tough call between Cinderace and Inteleon. An agonizingly tough call. Ultimately I think Inteleon is a little better without access to the Hidden Abilities. Though if the hidden abilities were available, Cinderace would win by a long shot.