My two "holy crap" wire moments came (1) from moving from generic lamp cord (or equivalent) to Kimber Monocle XLs for speaker cable and (2) replacing the stock wires on a pair of Sennheiser HD600s with Cardas wire. (Not that either is an "ultimate" in and of itself.) Everything since then has involved trade offs and increments, but those two were definite night and day moments.
