Megadeth was, and probably still is, a 4-piece speed metal band. Dave Mustane, the singer and lead guitarist, used to be in Metallica.
They released quite a few albums and a few singles, but didn't really seem to do very well.
I know "Hangar 18" was in the charts when I was about 16, and I know "Symphony of Destruction" was quite good, but I can't think of any landmark things they've done.
I never understood why the Metal charts had loads of metal in them when the normal charts didn't. Surely they'd have to be in both for the charts to be accurate.
Maybe not tho'.