1. Magneto vs. Gandalf
That's right, it's what you'll all been waiting for. An Ian McKellen-on-Ian McKellen fight to the death. Hope you brought popcorn. So here's the deal. Magneto's superpower is all about the manipulation of metal, which thanks to Tolkien's healthy distaste for industrialization, is largely absent in the Lord of the Rings world. Maybe if he got lucky there'd be someone with a sword or mithril around, but since Gandalf has a wooden staff, he might be out of luck. Then again, Gandalf doesn't seem to really excel at one on one battles. He barely escaped Saruman, and that dude's ancient. So maybe Magneto, with all his wiliness and cunning, would be able to scrape together a win.
Who Would Win: Who am I kidding? Gandalf would just summon his magical eagle of convenience like he always does, and fly away into the sunset.