None of the starters are viable in Nat Dex AG. If you're accounting viability in Nat Dex OU, then it is Greninja-Ash > Rillaboom > Swampert-Mega > Charizard-Mega X & Y > rest unranked.
In case of Nat Dex AG, none of the starters are viable a bit. However, Cinderace, Mega Blaziken, Mega Blastoise have been banned from Nat Dex OU and hence can be considered the most viable.
When speaking in general, low ladder kids love their Cinderace so much. But, really it's terrible and the closest thing you can use is Mega Blastoise, though it's still niche to use over M-Ray.
Tl;dr Mega Blastoise. If not accounting Megas then Cinderace and after that Ash-Greninja.