Nowhere does Tolkien provide a detailed description of Sauron’s appearance during any of his incarnations.
According to The Silmarillion, Sauron was initially able to change his appearance at will. In the beginning he assumed a beautiful form, but after switching his allegiance to Morgoth, he took a sinister shape. In the First Age, Gorlim was at one point brought into “the dreadful presence of Sauron”, but the only concrete hint about his appearance is a reference to his daunting eyes.
As part of a plan to destroy Huan, Sauron took the form of the greatest werewolf in Middle-earth’s history. When the plan backfired, he assumed a serpent-like form, and finally changed back “from monster to his own accustomed form”. The implication is that his “accustomed form” was not, at least, overtly monstrous. It is understood to have been humanoid.
Sauron took a beautiful appearance once again at the end of the First Age in an effort to charm Eönwë, near the beginning of the Second Age when appearing as Annatar to the Elves, and again near the end of the Second Age when corrupting the men of Númenor.
One version of the story describes, in general terms, the impression Sauron made on the Númenóreans. He appeared “as a man, or one in man’s shape, but greater than any even of the race of Númenor in stature… And it seemed to men that Sauron was great, though they feared the light of his eyes. To many he appeared fair, to others terrible; but to some evil.”
Like Morgoth, Sauron eventually lost the ability to change his physical form (his hröa). After the destruction of his fair form in the fall of Númenor, Sauron was unable to take a pleasing appearance or veil his power again. Thereafter, at the end of the Second Age and again in the Third, he always took the shape of a terrible dark lord. His first incarnation after the Downfall of Númenor was extremely hideous, “an image of malice and hatred made visible”. Isildur recorded that Sauron’s hand “was black, and yet burned like fire…”. Gil-galad perished from Sauron’s heat.