At the end of the day it depends what kind of thrash you prefer... Brutal? Melodic? Progressive? Raw? Technical? Good production? Bad production?
Vektor's Black Future has a bit of everything (except bad production), and is certainly the most innovative album I've heard from the new wave of thrash.
Also try Mutant, they don't have a full-length album yet but their 4-song EP released last year is up there with anything from the new wave (other than Vektor who are totally unique)...
And if you like that, you may also like this:
They're Serbian, and the singer is dead now, but the more I listen to them, the more I like them.
For older, melodic, slightly technical and underrated thrash I recommend the following:
Vendetta - War (from the album Brain Damage)
Paradox - Heresy (from the album Heresy)
Both bands are German but heavily influenced by the American thrash scene.
My favourite of all thrash albums is Forbidden's Twisted Into Form. It's dark and technical with spiralling tech riffs, great clean sections, Paul Bostaph's amazing drumming. Russ Anderson's voice is superb and ranges from a great low, meaty register to ear-splitting high notes...
They are my top recommendations, but as I say, it depends what kind of thrash you like. If you want something more brutal try Demolition Hammer. There is something majestically wonderful about their Tortured Existence album. The song titles crack me up, and their riffs are fucking deadly.
If that's the sound for you then try Devastation's Idolatry, Sepultura's Beneath The Remains, Exhorder's Slaughter in the Vatican or any of the other albums produced by Scott Burns around that time.
This is from the perspective of someone who likes melodic thrash, I'm sure some people here will say I'm lame for liking that stuff but to be honest I couldn't give one single tiny flying foetus what anyone else thinks of my music taste, and neither should anyone else...
I actually made an Amazon list a while ago with some of my favourite thrash albums, check it out for more recommendations:
http://www.amazon.co.uk/The-Best-Beasting-Thrash/lm/R1TDXXAHXA35JQ/ref=cm_lmt_srch_f_1_rsrsrs0