i am not a big fan either, but to say the beatles are overrated is kind of like saying johnny cash or led zeppelin is overrated. the beatles changed music.
Metallica did stuff for Rock that simply wasn't popular at the time. They didn't sound like hairbands and they didn't sound like 60's bands like Zep either. They did stuff no other band did.
I have a correction...when I said Green Day earlier I meant modern greenday. Pretty much anything after Dookie/Warning is garbage. Anything prior to that is good stuff.