Ride The Lightning or ...And Justice For All. I can see how, from certain more objective viewpoints, Puppets could be considered their tightest, most focussed, most defining album. But that's precisely why I enjoy the youthful blaze or Ride and the epic exploration of Justice so much. They lean out to other corners.
I voted for Kill 'em All, although the composition and the songs on RTL are probably better, Kill 'em All is more direct and probably is the album that really got me into Metallica.