A New Hope would have only been moderately successful at best without the combination of David Prowse doing the physical work and James Earl Jones voicing Vader. Possibly a flop.

Yes, the rest of the cast was solid and it would have still been a good B movie without them, but the voice and physical presence of Vader set the tone of what the protagonists were fighting against. Vader isn't the best character either! In fact he is a one note villain in A New Hope.

The combination of both actors was the secret sauce that set the foundation for the entire series. Heck, Vader overshadowed Palpatine in their scenes together for me, even with Ian McDiarmid's excellent performance.