Just how powerful was Gandalf the Grey ?

[SilverToungedDevil]

In the chapter "Many Meetings", Gandalf is talking to a recuperating Frodo and says :

"Yes, I, Gandalf the Grey," said the wizard solemnly. "There are many powers in the world, for good or for evil. Some are greater than I am. Against some I have not yet been measured. But my time is coming. The Morgul-lord and his Black Riders have come forth. War is preparing !"

Who do you believe he is thinking of at that time ? Obviously he means the Morgul Lord and the other Nazgul ( evil ). He must also be making some reference to the Elven Lords ( good ). While Sauron must be in the " greater than I am " category, who else does that leave ? Where in the ranks of the great and powerful does Gandalf the Grey stand ? ( I'm only talking about the Powers of middle-earth, I'm not including the Valar. )