http://www.smogon.com/forums/threads/sun-moon-in-game-tier-list.3587886/reply?quote=7135653Popplio is by far the best starter. I've been bugging ColonelM to move him to top. He is the only starter who has demonstrated the ability to completely solo the game, including the Lurantis and Vikavolt fights:
https://www.twitch.tv/eekcast/v/107262403
You can nickpick about Popplio's level being high during some fights (it's the nature of using just one Pokemon) but until somebody shows me video evidence of any other Pokemon soloing the game, Popplio is the undisputed king in my books.
Any pokemon can solo the game when they're overleveled. Popplio's a solid pokemon, but it isn't S tier. In your average efficiency run, it's going to struggle against more than a few major boss battles and especially early game until you get Scald. You would have to be hideously overleveled to solo Ilima, the totem normal, Hala, Totam Lurantis, Totem Vikavolt, and Totem Mimikyu. Those fights are much better done with team mates to help out, and Popplio's early game slog is what keeps it from being S-tier.