Alolan Vulpix. Would be a change from the Fire/Water/Grass start and would be perfect against Brock(x2) and be able to fight Misty after considerable training to learn Dazzling Gleam or import one with Freeze Dry ^_^. It’s Kanto variant would be my pick if Alolan variant is off limits. Honorable mention to Sandshrew and it’s Alolan variant(Alolan Sandshrew would be a strong pick too).
Kanto-only: Eevee worked out pretty well for me, but I'd like to have a Ditto ride on my shoulder. c: All happy and blobby and cute. (Then you throw it into battle, and it turns into anything the Pokemon world has to offer and rains 5 Power Points of death upon its foes! >:D)
If we can choose from all Pokemon...Oh man, definitely Oshawott!!