Which of the gods was often called the invisible god?

Ptah

Sorry, Miles. The answer is Amun. Amun was the creator god and was often referred to as the invisible god for he had never manifested himself before man. However statues of him depict him as human, or human with the head of a ram.

Ptah was also know as The Great Invisible One

I can't find a reference to that, Miles.Can you supply one?
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Amun

Originally, he was simply nothing more than a deification of the concept of air, and thus wind, one of the four fundamental concepts held to have composed the primordial universe, in the Ogdoad cosmogeny, whose cult was strongest in Hermopolis. His name reflects this function, since it means the hidden one, reflecting the invisibility of the air, and of the wind. Like all other members of the Ogdoad, his male aspect was usually depicted as a frog, or frog-headed. Symbolically, invisibility was represented by the color blue, since it was the color of the sky, seen through the air, and so this was the color usually given to Amun's image. - Wikipedia

Amun, whose name means the Invisible One, was usually depicted as a man wearing two tall plumes on his head, and holding a sceptre in his hand. - BBC