I'll provide the opposition for the Megadeth bandwagon (this is only my opinion, please don't be offended):
1) Megadeth only have one really great thrash album from back in the day IMO, Rust In Peace, the rest have some great songs, but have equal amount of meh songs on them, even Rust In Peace seems to have a bit of filler on it.
2) Most of Megadeth's hard rock stuff can become annoying easily, due to Mustaine singing like his nose has been stapled up.
I personally wouldn't be able to stand only hearing Mustaine's voice for the rest of my life.