As for the original topic, the OP's preferences are not only defensible, but have a solid precedent: The Lord of the Rings. Gandalf is the only one who can deal with the Balrog and Saruman, and has a better track record against the Nazgul, but the other heroes accomplish much more against wargs, orcs and trolls. It's not a hard and fast division, though--Gandalf can handle mundane foes, and we all know about the Witch-King. In addition, it's Aragorn, not Gandalf, who bests Sauron in a battle of wills.
Except for one slight problem--Gandalf isn't really a wizard. He's a celestial being, as are the Balrog (albeit corrupted and lessened) and Saruman (with the same limits Gandalf had).