Route: Chansey I mean. He'd pick something that would be easy to keep an eye on, unlikely to be able to get into too much trouble, not psychic so that he doesn't have to deal with Sion telepathically nagging him, and cute so that he could tease Sion about it later. My first instinct says Lillipup, honestly, though other options might be Eevee, Meowth, Purrloin, or Poochyena.

Camp: Assuming that he had all the species and their pictures and attributes listed in front of him, Camp!Ryner would actually probably pick Espeon or another small psychic-type because then Sion could talk via telepathy. It's not optimal, sure, head-voices are still not really a good thing, but it's better than not being able to communicate at all.

And you know what, I like that question, so reverse for either or both of your Sions.