Kill 'Em All through the In Justice era is unmatched in totality, but after that, Metallica fell off. And Megadeth's best days ended (imo) after Extinction (or maybe even Rust in Peace). So if you're talking the whole span of their careers, both are heavily flawed. But if you're talking in their prime, it would have to be Metallica because it was one thrash masterpiece after another. Although Rust in Peace is as good as anything Metallica released between Kill 'Em All and Justice.
But nowadays? The latest Megadeth record destroys modern Metallica. And by a large margin, imo. I don't know how Dave managed to put together such a mean sounding thrash album at his advanced age, and while suffering through cancer, but he did. Metallica, on the other hand, are busying playing with Hanna Montana and Lady Gaga while James is (literally) crying on stage about his insecurities which is just... sigh.







