Couple that could make the shortlist:
Abercrombies Bayaz, First of the Magi.
Brooks, Allanon, Last of the Druids.
I cast my vote for Gandalf.
With regards to Gandalf, posters above have mentioned that if Gndalf acted then it would be a Deus Ex Meachina type of intervention but we know that isn't the case. While Gandalf is very strong (one of the strongest beings left on Middle Earth) he was utterly terrified of the corrupting power of the Ring and he could not have stood against Sauron without the power of the One Ring which he ultimately feared would corrupt even him. Worth remembering that Galadriel was the most powerful being in Middle Earth (I suspect even more so that Sauron) and even she does not trust herself with the One Ring.
The true test of Power here comes not from blasts of magic or fireballs but from resistance to the corrupting power of Men.
At least that's how I see it with regards to Gandalf - he did everything he could, within the limited role he could take on Middle Earth. I never saw this as Tolkien restricting Gandalfs interaction but as Gandalf himself limiting his own powers for fears of repeating mistakes of others.