Up until recently I would have answered BAY AREA without hesitation! Now I would still say BAY AREA but I would feel very guilty towards Canadian excellences such as INFERNAL MAJESTY (put an Umlaut somewhere) and German born but Oakland sounding musos VENDETTA!
The second lp by Vendetta and the first one by Infernal Majesty are the best thrash records I picked up in a very long time!
Did I tell you that in July I went to a (mostly thrash) metal fest in Brescia called Colony Open Air that featured Death Angel, Exciter, Kreator, Demolition Hammer, Asphyx, Loudness, Hell and Wintersun (dunno why they threw them in the lot). Italian heavy metal band Skanners was also in the bill, but I came too late to see them. Very good festival overall. Demolition Hammer were the best, I'd say.