Learn Italian: Entrambi i grossisti offrono un ottimo servizio. - Both wholesalers offer an excellent service.

In Italian, the word entrambi (both) must always be followed by the definite article of the noun it modifies. While in English we simply say "both wholesalers," Italian requires the equivalent of "both the wholesalers" (entrambi i grossisti).

Grossista ends in -a, so why is the plural i grossisti instead of le grossiste?

Words for professions ending in -ista can be either masculine or feminine in the singular (il grossista or la grossista). Because these specific wholesalers are masculine (or a mixed-gender group), the masculine plural rule applies, changing the ending to -i to make i grossisti. If they were an exclusively female group, it would indeed be le grossiste.

Why does the adjective ottimo come before the noun servizio, and could I say molto ottimo?

While descriptive adjectives usually follow the noun, placing them before the noun adds emphasis and a subjective tone. Because ottimo is an absolute superlative (meaning "excellent" or "extremely good" by itself), it already contains the idea of a maximum degree. Therefore, you can never say molto ottimo.