Why did Olorin seem the least of the Istari?

...Or, Sauron possibly didn't know that Gandalf was Olorin, though I kind of doubt this.

True, Olorin was the wisest of Maiar, but he wasn't the most powerful. Curunir (Saruman) was more powerful than he, and he came to Middle-earth first, so he was appointed head of the White Council. Not to mention that Gandalf keeps travelling from place to place (hence the name Grey Pilgrim), and, as Legolas said, Olorin was humble, I wouldn't say underestimated himself, but was cautious or wary of himself, because he knows his powers and limitations.