I don't know if my outlook is biased, so, the best metallica album was master of puppets, this album break all the music molds , the follows albums was made with the money in mind not with the heart.
Personally, its too close to call between MOP and Ride the Lightning- I have nothing to say about anything after Jason Newsted left the band. I mean they were making- in my opinion- their best stuff before Cliff died (RIP!). Jason kept the band rockin'- but my heart has wandered since he left... Also, the only thing I can say was a benefit to them making Load and Reload is Kirk Hammet's hair- he looked way hot without the poodle doo!
Honestly its what mood your in. I play guitar and i know most every song and they cover more spectrums of music than the beatles. I think Justice was there best, lars killed the drums the whole album, i mean that was there peak playing ability and writing. master of puppets is amazing the solos fit the songs perfect and ride the lighting has more energy in it than anything, kill em all was just so different from everything, so raw. and yes i can see why you all love master of puppets the most b/c like ozzy said, he created metal, and metallica redefined it with master of puppets. But all there albums are awesome, even there newer ones after the black album. and there tuning back to E for the next album so im excited.
I disagree with you there about the most recent albums being made with the money in mind. Metallica are notorious for being totally anal about their discs when they're in the studio, they are all about the music! They don't churn out a new disc every 10 months like KISS used to.... THAT was done with money in mind!!