Testament lack the sheer ferocity I look for in thrash (both 80s style and modern) metal. They are a decent band, and Alex Skolnick is a great guitar player, but somehow they lose some power in their riffing that I find much better in other bands.
Then again, I'm quite picky when it comes to thrash, I like what I like and dislike what I don't. I prefer east coast thrash (Anthrax, Overkill, Nuclear Assault)...and I definitely prefer German thrash (the big three: Sodom, Kreator, and the aforementioned Destruction).
All the bands I listed I prefer over Testament; just my preference.