its a top 50 out of about a squillion releases: i.e. THEY ARE ALL ESSENTIAL LISTENING.
but specifically to some of your queries:
Raven - not thrash metal - NWOBHM
Exciter - Perhaps could be considered thrash metal, but i dont care for them anyway and neither did any of the contributors. we cant cover everything and everyones taste
atheist - death metal maybe?
artillery > megadeth - i think not. a matter of opinion, but 99 times out of 100, you will be out voted. so im not too unhappy with the order.
i also dont see the problem with having more than one album from the best thrash bands of all time on the list, but hey thats just me. i dont see the point in taking stuff like south of heaven off, just so a lesser album can get some publicity.
plus im betting most of the people who have posted here havent even heard all of the albums we listed or 50 thrash albums ever for that matter.