MetallicaSlayerAnthrax, and Megadeth were universally the big names in Thrash Metal.  Following closely behind them were Testament.Vocalist Chuck Billy and guitarist Alex Skolnick were becoming bigger names all the time.  They still remain as one of the genre's founding fathers and Thrash eventually morphed into today's Metal that still practices some of the same elements.

Ironically they suffered a similar fate as the Pop Metal bands of that era did even though they had nothing in common with those bands.  By 1992's "The Ritual" they already showed signs of wavering.  They returned to a more powerful form in 1994 with "Low" and since then have remained a relevant band in the world of Metal.

Testament "Practice What You Preach" from the 1989 release with same name.