As I stated, which type (water, fire, etc.) of Pokemon make up the majority of Pokemon in the latest games, Sun and Moon?
Counting manually using Bulbapedia's Handbook Pokedex, Using the 'Refine' function to restrict the list by type, Water has 60 entries in Sun/Moon, higher than all other types. Flying comes in second at 50.
This includes different formes of the same Pokemon that may share a Pokedex number as well as event exclusives.
This also holds true for the National Pokedex. According to Bulbapedia's entry on the Water type:
As of Generation VII, there are 133 Water-type Pokémon or 16.6% of all Pokémon (counting those that are Water-type in at least one of their forms), making it the most common of the eighteen types.
This is followed by Normal type in 2nd place:
As of Generation VII, there are 109 Normal-type Pokémon or 13.6% of all Pokémon, making it the second most common type after Water.
As for the least-used type, that would be Ice:
As of Generation VII, there are 40 Ice-type Pokémon or 5.0% of all Pokémon (counting those that are Ice-type in at least one of their forms, including Alola Forms), making it the rarest type.