Luckily for me, I am a born and raised metalhead so I am well aware of Carcass and Testament. Carcass was great up to Swansong, though from an unobjective view Swansong wasn't bad musically, just not mega heavy. Testament how ever are legends. I liked all the albums except demonic. Alex Sklonick is amazing at guitar, and Eric Petersen isn't bad either. I hope that one day I can play even half as good as either of them haha...
Metallica I think lost it after Master of Puppets, and Ride the Lightning, but the funny thing about Megadeth is all of thier best material was anything pre Youthanasia. But which guitarist came in on Youthanasia? One mr Marty Friedman.