[Zigûr]

It's worth remembering that the shadow fell upon Númenor long before Sauron was ever brought there. He was responsible for the Melkor-religion and the nadir of Númenor's darkness, but only because he exacerbated and took advantage of a long history of increasing faithlessness which had begun during the reign of Tar-Ciryatan over a thousand years earlier. They had forsaken the Valar long before Sauron ever arrived.

So we can look at it two ways:
a) what if the Númenóreans had never fallen to the shadow whatsoever?
b) what if Ar-Pharazôn had refused the counsel of Sauron?

Regarding a) I think that there would have been inevitable war for mastery of Middle-earth between Sauron and an alliance of the Noldor and the Númenóreans. Gil-Galad had foreseen the need for this as early as the reign of Tar-Meneldur. Tar-Minastir came to the aid of Gil-Galad. His son was Tar-Ciryatan. Perhaps if the shadow had not fallen, Ciryatan would have followed his father's example and established close ties with the Eldar. That being said, it was under the shadow that Númenor grew from a strong nation to a military and imperialist superpower, so a war between such opponents might have also been far more bitterly fought than, say, Ar-Pharazôn's bloodless rout of Sauron's hosts at Umbar.

As for b) I don't think it would have mattered much. Sauron had the One Ring with him, and I fear that his domination of the hearts and minds of the Númenóreans at that stage was largely inevitable.