There are a few who could give Gandalf a decent fight as Gandalf the Grey.  (Elrond, Sauron, Saruman, Galadriel, and some really ancient beasts like the Balrog)  As Gandalf the White, I'm thinking he wins all of those fights, most without a decent battle.  I do like your "most powerful in his own fictional universe" argument, though we do have to imagine some sort of Superfriends scenario for this fight to happen.  (like how I pulled in your example?)

What about Mr. Tom Bombadill? Somehow I have got the impression that he is pretty much a supreme being, even too awesome for the films.

edit: but maybe he is just too much of a hippie to even want to put up a fight...

Gandalf can come back from the dead and still kick ass
Dumbledore is just dead...

Dumbledore could shout at Gandalf from his painting though!

Gandalf would win. Why? Read this: http://www.newsweek.com/id/50787

That is just ignorant, take you bullshit somewhere else.

For example, Dumbledore would be able to kill/petrify/etc Gandalf with a single spell, whereas Gandalf's spells don't work like that... At least not on the movies, I haven't read the books yet.

There are a few who could give Gandalf a decent fight as Gandalf the Grey.  (Elrond, Sauron, Saruman, Galadriel, and some really ancient beasts like the Balrog)  As Gandalf the White, I'm thinking he wins all of those fights, most without a decent battle.  I do like your "most powerful in his own fictional universe" argument, though we do have to imagine some sort of Superfriends scenario for this fight to happen.  (like how I pulled in your example?)

I do believe that you have convinced me.. In a clean fight (and without tweaking the books or characters abilities) Gandalf would in fact be more powerful. I have to take your word for it, and I will do so.