I voted for Metallica - based on the first four albums by each band. (I didn't buy any of the albums they both released afterwards.) "Ride the Lightning" was the third Metal album and the first Thrash Metal album I bought and I think I listened to it about once per day for more than a year (way back then). I listen to it rarely nowadays, but I still think it's Metallica's best album and one of the best Thrash Metal albums ever. I definitely like it better than any of Megadeth's first four albums, whereas I'm not so sure as regards "Master of Puppets", and even less as regards "Kill 'em All". But I can safely say that I like any of the first four Megadeth-albums better than "AJFA" which is - by far - my least favourite Metallica album (of the ones I own). I did enjoy "One" and maybe a few other songs, but I don't like the sound on this album and Jason's non-audible bass-playing was perhaps (as I might have mentioned elsewhere before) the biggest disappointment (considering how he had sounded on "Doomsday for the Deceiver").
I find it more difficult to rank Megadeth's first four albums than Metallica's first four, though "Peace Sells..." definitely got the most spins of them.