I'm going to ram my head into the wall every time I see another "Gandalf vs. <Some other wizard/magic-user>" discussion. Seriously, I can't stand them. Generally for phenomena that exist purely for fun I can put aside the logical quibbles I have with them, but this one is just so.... so fundamental I can't help but be driven crazy by it.
To every fanboy out there: Unless you are arguing about characters within the same mythos (and preferably written by the same author - looking at you here, Star Wars fans) you have no basis for comparison. I can't tell you whether Gandalf would win in a fight against Voldemort, because the entire fundamental rules of their realities are different.
If I'm being glib I'll just pick the character I like better, but if someone were to - in all seriousness - ask me to decide the question, I'd demand a lot of money for all the time I'm going to need to spend constructing the logical/physical/magical framework that translates one world into another. Most of that would be pulled straight from down under, and thus the decision is still arbitrary.
Technically the same applies even to mundane warriors, though at least there you can kind of trust that they need to follow basic physics. However, given the prevalence for fiction authors (myself included) to have their characters do some pretty improbably stuff, they become just as impossible to compare.