We probably shouldn't start the whole Metallica vs Megadeth thing again, but it's just too much fun
Three words: Rust In Peace
One word: Youthanasia.
The "mainstream era"? Two words: Cryptic Writings
But I think I prefer Metallica to Megadeth nowadays. Both have their ups and downs, but Metallica's best albums (MOP and Load) beat every Megadeth album.
Not even close IMO. I can think of at least three 'deth albums (RIP, CW, Endgame, maybe even Countdown and The System Has Failed) that beat Metallica's "masterpieces" (AJFA and MOP).
Metallica is and was a very important band for me, since they were the first band I was really a fan of and they introduced me to metal. I used to say that unlike them, Megadeth still put out great albums, but after 13 and Super Collider, that's unfortunately not the case anymore.