I'm not one of those guys that's like "holy shit Metallica is a sellout!", I don't think the Black Album was, but they surely got rid of their thrash metal roots in a favor of a more accessible sound
But yeah, they made AJFA and never recovered, it's their most complex and progressive album and it's my personal favorite. Ride The Lightning is also a masterpiece, Master of Puppets is considered the quintessential thrash album, but I still like AJFA more