Maiden and it's not even close. Maidens only low point was the 90's and even that is better by most bands standards. Metallica stopped giving a shit in the mid 90's and has never been the same. Even in their sixties Iron Madien are all around better than Metallica. Lars is a HORRIBLE drummer, Kirk somehow regressed as guitar player so badly that he struggles to play the same songs he's been playing the past 35 years. Hetfields voice sucks, but he's still amazing at guitar. Trujillo is great, but looks out of place on stage and has horrible backing vocals.
Iron Maiden still puts out amazing records, while Metallica puts out garbage like Lulu. Metallica checked out a long time ago.