For competitive play, either Tapu-Lele, Tapu-Fini or (if including USUM), Dusk Mane Necrozma. Zeraora seems like it'll be up there too if including mythicals.
If talking about for a story playthrough, Solgaleo and Lunala (as well as Necrozma in USUM) are the only ones you can get in the events of the main story unless having the Zeraora event, so any of them will do. I'd say Solgaleo for Sun and Dusk Mane Necrozma for Ultra Sun are the best due to their variety in move types, though that's assuming you're not playing Moon/Ultra Moon.