Metallica were good up to the point in the career when they went up their collective assholes (Read:Load/Reload/Napster/St. Anger). Megadeth have been doing Megadeth stuff for, well, forever. They're like Irom Maiden in that their sound rarely changes over the years but what they do put out is consistently high quality. Ain't broke, don't fix. Anthrax are probably the most successful of the "experimenting" old-school thrash metallers. Their later stuff (post-We've Come For You All et al.) is hugely different to their Among The Living era, but it's still very good and still very 'Thrax. Slayer...well, what can we say about them? Hugely campish and silly and they probably know it, Slayer started out cool, became a joke, then seemed to embrace it in a way and have just gone with it. Kinda like a post-modern Thrash metal band. They don't take themselves too seriously.

Man I'm not even close to putting them in any order and I've hardly touched on the music yet...