The Rat which launched 1,000 bands.. the ProCo Rat is one of the originals, often cloned, seldom beaten.
The original used an LM308 but these are becoming un-obtainium, and modern releases use the OP07. Other single op-amps can add different flavours, TL071 for example.
Experiment if you like that kinda thing, or just use an OP07 like I do.
