I was a Metallica fan when Master of Puppets was their latest album, shortly before 'And Justice for All' came out which was in sorely need of proper editing and remixing.
I thought "... And Justice For All" was quite unique. It was more darker, it dove more in to social lyrics. Musically was way heavier, and I though the time changes on every song were very extreme but fun and interesting to listen. Its not Metallica's best album because of the bass it seemed that it was left out. "Master of Puppets" I think was their best album, as far as having best all around songs through out the album, and being consistent. Although I think their best song and my favorite of all time was Fade to Black.
Megadeth's best albums.... well its tough to decide. "Peace Sells... But who's Buying?" had a very thrashy feel, while being musically great. It was a great combo of thrash with harmonies. "Rust in Peace" might have been their best. It wasn't all high speed like their sophomore album was. It was a good mix of low, mid, and high tempo songs with great detailed harmonies, solos that were off the chart, and that gave to me a jazz like sound, while keeping it a bit heavy. The bass and drums were impeccable, and Daves vocal skills were much improved over previous records.