Bah, you're all phillistines! I pick REAL thrash metal like....Acid Reign, Slammer or Onslaught
*snigger*
Only joking!
I meant Xentrix!
*guffaw*
No, but seriously, I mean Sabbat! (UK), especially the third album!
*rolls in the aisles*
They were a little later, and not strictly pure thrash, and didn't receive much in the way of popular support, but Skyclad are my favourite British "thrash band", for what it's worth.
Of the "big" bands, I dig Exodus the most as the 5th, mostly because they kept their own beat, and didn't try and sound like anyone else. Testament, much as I like them, always seemed to me to be trying to sound like other bands. Initially Megadeth, then Metallica. I still like 'em though...