In your detailing regimen? For me, it would have to be QEW. I love being able to wash my car in the winter. I love being able to wash my car without stringing a hose out across the driveway. I love being able to follow it up with a QD (my second essential item) and have it look freshly waxed.
What detailing supply could you not live without?
What detailing supply could you not live without?