Let me quote The professor here:

1)«Wisest of the Maiar was Olórin. He too dwelt in Lórien, but his ways took him often to the house of Nienna, and of her he learned pity and patience.»
2) Cirdan said he was «the greatest spirit and the wisest»
3) He was a Maia of Manwë, the king of the Valar.
4) He is by that time, the chief of the Istari.
5) Tolkien's description of his effects over the Uruks of Saruman speaks for herself.
6) He was resurected by Eru himself!

Olorin's powers were far more important than what Gandalf shows in middle earth for many reasons: his wisdom, his mission (give courage to the free-people, not fight at their place), his Istar form, etc.

Tolkien actually gave enough hints about Olorin's true nature. Some even think that he was Manwë himself. Tolkien denied that, but still. No way a wraith could defeat the greatest of the Maiar. Gandlaf didn't defeated the WK because Manwë didn't include that in his job description x)