For the main story, Charjabug's evolution, Vikavolt, works great unless you run into someone with a type advantage (fire or rock), although it doesn't have much for speed. Magnezone is much better statistically, but is extremely weak to ground and doesn't fare well against fighting type moves, though it loses its rock type disadvantage. Magnezone is more resistant to other types as well.
If you only want to use Alolan Pokémon, Charjabug/Vikavolt is the obvious choice, but Magnezone would fare better against a great many attacks. I'd go with Magnezone for more speed and resistances. Keep in mind that both these Pokémon are at their best when confronted with a Pokémon that has a type disadvantage. And watch out for ground types, as Salandit and Magnezone both have a big weakness to this type.
