If you want to base an opinion on their first five albums then I agree with you. No one has done what they have.
That also makes me correct in saying they were the greatest for a period. If you want to base it on an entire history or catalog
then I would say maybe Pantera. They put out some pretty heavy shit that might equal what Metallica did. My top answer
for overall consistancy and longevity would be Megadeth. They (in my opinion) have yet to release an album that is utter crap.
You even stated they were better than Metallica on the Big Four so that tells me they still have it. I'm going to see them next
month and I can't wait. Lastly, "Any" metal band putting out good albums is better than current Metallica.
I'm not bashing Metallica. Up to a certain point I thought they were great and like you state maybe the best "American" metal
band out there. All I'm saying is that they're not so great anymore.