At the risk of sounding old here.... Gotta go with the OG Demonoid in its day. It had it's downsides and could be a bit sketchy at times. But my oh my was it comprehensive in its prime. One could find almost anything on there.

Others that come to mind include What.CD (Although RED has filled the void nicely), Black-Cat Games, and Cosa Nostra (Fun Community).

Lastly shout out to FTN, although it was probably the most overrated tracker ever. However I have to admit it was probably the most memorable invite I ever received.