Ian McKellan
Im going to say it, Ian was not the best choice for Magneto. While being a great actor he was no Magneto. Fassbender has the physical presence to play him which makes him the better of the two.
Clint Eastwood would have been much better in the role. That man is stoic and thats what Magneto is.
I did still enjoy Ian but, physically he didnt fit the role.