Low, Demonic (which I know is only Eric) and The Gathering are the shit. I don't care about The Formation of Damnation it was very mediocre and just an accomplishment for them to put out a record after all that time.
Alex Skolnick has always had a part in songwriting (at least, all of the solo guitar work, and his solos are definitely THE BEST in the whole USA thrash metal scene :P). In their last album, he has credits in many songs and he has written entirely Dangers of the faithless, IMHO the best song off TFOD.
Murphy is God (2)... well I'll say "a God"... their best shit are the 2 albums and live record he's on anyway. I like Skolnik but think people make a little too much of him. I met him and he was a little arrogant and it's not like he was ever really a WRITER in Testament it's always been Eric.