Ride the Lightning also a great album. No argument from me.
Revisiting these albums and feeling a little anti-climatic with it .. I don't know. I think words like groundbreaking are a bit of a cliche but when those albums were first released, at the time, yes, the whole thing was blowing peoples minds, it was such a new sound coming through. I don't even think I have done it justice summing it up in that sentence, to be honest.
Personally, I wouldn't say MoP is the undisputed metal album of all time, but all of those early Metallica albums have rightly deserved their top spots in metal history.