Why did Olorin seem the least of the Istari?

[The X Phial]

I don't. I think that it is quite possible that Sauron had no inkling that the Istari were Maia, at least until Gandalf came back as the White.

I disagree. When Saruman used the Palantir he was likely read by Sauron. I would be surprised if he DID'T know that Saruman at least was an ancient spirit. Most of Saruman's will to resist Sauron by that point was currupted by his own desire for the ring.