It doesn't really matter, as it is postgame. Starters are all pretty even statwise and battle wise already, so there really isn't much reason to just choose who you want. They start at lvl 5 anyway, so it would take a long time to get on your team or battle ready.
Like x said, you could pick whichever starter is less popular, which would probably be Chikorita, Snivy, and Piplup.
All these Pokemon are available in the same method as getting the hoenn starters