I personally prefer McKellen. Even in some of the lesser X-Men movies like The Last Stand, he singlehandedly manages to be totally brilliant, capturing not only the character's cruelty and intimidating presence, but also nailing his more tragic, vulnerable, and even sort of charming side with pinpoint accuracy. It's easily one of the good sir's finest performances in cinema IMO. I like Fassbender too, but the films' writing got kinda sloppy in Apocalypse and Dark Phoenix.
"All those years wasted fighting each other, Charles...to have a precious few of them back."