Quite a lot of bands have already been mentioned, yet there are still some bands and albums I´d like to add:
Détente - "Recognize No Authority" (www.fear-of-god.com)
Fear Of God - "Within The Veil" (not a "classical" thrash metal album, but check out the mp3 of "Drift" on www.fear-of-god.com), "Toxic Voodoo" ("thrashier" than "Within The Veil" with a slight death metal touch as far as the music is concerned, but personally, I definitely prefer "Within The Veil" whose atmosphere and vocals are far more intense)
Znowhite - "Act Of God"
Agent Steel - "Skeptics Apocalypse"
Voivod - "Killing Technology" (IMO the best release of their thrash metal phase, but you might also check out "War And Pain" - it was recently re-released in a 3 CD-box -, "Rrröööaaarrr" and "Negatron")
Mekong Delta - "Mekong Delta", "The Music Of Erich Zann", "The Principle Of Doubt" and "Dances Of Death"
Hexenhaus - "A Tribute To Insanity" (still their best IMO), "The Edge Of Eternity", "Deja-Voodoo" (the last two are more technical, I´d say they tend to go a little bit in the direction of some Mekong Delta-stuff)
Toxik - "Think This"
Anacrusis - "Suffering Hour", "Reason" (not pure thrash metal, but they definitely had a style of their own; you can download all their albums - but the last two, "Manic Impressions" and "Screams And Whispers", have nothing to do with thrash metal - from their official website: http://apogee.whack.org/~ken/ )
As you already mentioned Watchtower, you should also try
Sieges Even - "Life Cycle" and "Sophisticated" (for more info go to www.siegeseven.com), their albums "Steps" and "A Sense Of Change" are also excellent, but have nothing to do with thrash metal, so I guess they are not what you´re looking for
As for Forbidden, also try to get hold of their live-EP "Raw Evil - Live At The Dynamo" - it features three songs from their debut album - "Chalice Of Blood" and their masterpieces "Through Eyes Of Glass" and "Forbidden Evil" - yet in a much rawer version (perhaps hence the title "Raw Evil"?) and a live cover version of "Victim Of Changes".
The final recommendation might be a bit off-topic, because though some songs of this band (at least on their last two albums) also have thrash-metal influences, it is definitely not a typcial thrash metal band. I´m talking about Atrox. Among the mp3´s on their official website (www.atrox-web.com), the song "Flesh-city" is probably the one with the most thrash-metal influences - music-wise (the vocals, which most people tend to dislike, are something very different). Well, I think thrash metal purists won´t like this song. But listen and decide for yourself!
Détente - "Recognize No Authority" (www.fear-of-god.com)
Fear Of God - "Within The Veil" (not a "classical" thrash metal album, but check out the mp3 of "Drift" on www.fear-of-god.com), "Toxic Voodoo" ("thrashier" than "Within The Veil" with a slight death metal touch as far as the music is concerned, but personally, I definitely prefer "Within The Veil" whose atmosphere and vocals are far more intense)
Znowhite - "Act Of God"
Agent Steel - "Skeptics Apocalypse"
Voivod - "Killing Technology" (IMO the best release of their thrash metal phase, but you might also check out "War And Pain" - it was recently re-released in a 3 CD-box -, "Rrröööaaarrr" and "Negatron")
Mekong Delta - "Mekong Delta", "The Music Of Erich Zann", "The Principle Of Doubt" and "Dances Of Death"
Hexenhaus - "A Tribute To Insanity" (still their best IMO), "The Edge Of Eternity", "Deja-Voodoo" (the last two are more technical, I´d say they tend to go a little bit in the direction of some Mekong Delta-stuff)
Toxik - "Think This"
Anacrusis - "Suffering Hour", "Reason" (not pure thrash metal, but they definitely had a style of their own; you can download all their albums - but the last two, "Manic Impressions" and "Screams And Whispers", have nothing to do with thrash metal - from their official website: http://apogee.whack.org/~ken/ )
As you already mentioned Watchtower, you should also try
Sieges Even - "Life Cycle" and "Sophisticated" (for more info go to www.siegeseven.com), their albums "Steps" and "A Sense Of Change" are also excellent, but have nothing to do with thrash metal, so I guess they are not what you´re looking for
As for Forbidden, also try to get hold of their live-EP "Raw Evil - Live At The Dynamo" - it features three songs from their debut album - "Chalice Of Blood" and their masterpieces "Through Eyes Of Glass" and "Forbidden Evil" - yet in a much rawer version (perhaps hence the title "Raw Evil"?) and a live cover version of "Victim Of Changes".
The final recommendation might be a bit off-topic, because though some songs of this band (at least on their last two albums) also have thrash-metal influences, it is definitely not a typcial thrash metal band. I´m talking about Atrox. Among the mp3´s on their official website (www.atrox-web.com), the song "Flesh-city" is probably the one with the most thrash-metal influences - music-wise (the vocals, which most people tend to dislike, are something very different). Well, I think thrash metal purists won´t like this song. But listen and decide for yourself!