Either Pikachu, Rockruff, or Rowlet. Pikachu has lately got a lot of focus in Sun and Moon, especially with the Z-move. It may be possible that he will learn 10,000,000 Thunderbolt. Rockruff too has a chance to be the powerhouse. It too got a focus, and he have seen both Lycanroc forms, from Olivia, and Gladion. Most likely it will evolve into its Midday form or that rumored 3rd form. Rowlet has a chance, sure it doesn't really do much now but if it evolves into Decidueye, it may end up being one of Ash's best powerhouses yet. Litten may have a chance however, despite its backstory, it may only evolve into Torracat. This also happened with Tepig as well.