I voted Eevee because I think he'd be a cute and a fun pet to have... unlike Spinarak/Ariados who is creepy and Magikarp who is crapy. And how did you come up with Gyarados as a pet?
Most of those suck, but probably Vulpix. I wouldn't evolve her, because Vulpixes are too cute! And knowing my clumsiness, I'd rather not have Nosepass growing outta my head for a thousand years.