I would have a Caterpie! That guy is so cute! Plus on the early routes it can actually compete with those other weak pokemon.
Then it evolves into Metapod and I imagine strapping the cocoon to my back and talking to it as I carry him around.
Then you get a Butterfree! One of the happiest pokemon ever! I could also do this with Weedle because of how Beedrill is one of the coolest pokemon ever. Actually in my strange fantasy world, my spouse would start with Weedle and I start with Caterpie and we train them together!