Michael Fassbender
Keep Michael Fassbender as Magneto.
There are other Marvel villain characters, Doctor Doom or Adam Warlock, that are better suited for the other listed actors.
Magneto is the most popular villain in the X-Men franchise, and Michael Fassbender will most likely be replaced with a new actor for a third reboot of Disney's version of X-Men. Although many love Michael Fassbender as Magneto, it's still exciting to recast him.
Which actor do you think should play Magneto? Should Disney keep Michael Fassbender?
There are other Marvel villain characters, Doctor Doom or Adam Warlock, that are better suited for the other listed actors.
There are too many good actors to choose from.