Heh - I just can't leave the historical aspect alone - I just recalled this excellent track from the awesome "Court in the Act" by Satan, which is a proto-thrash epic with a nice complex structure predating "Ride The Lightning" and with more sophistication than "Kill 'Em All", but released in the same year as that earliest of thrash albums.
And this, which predates "Exciter" by 4 years;
OK - another thread required...