Straight up answer, as I think I know what the OP means, albeit hard to pin it down to just one thing. Will narrow it down as best as possible.
For PCPs I would say My own air supply, in this case it's my Altaros booster and my Chrono. Those two are invaluable to shooting my PCPs.
For Airgunning in general, I would have to give the nod to Good Quality Scopes. I would be dead on the water trying to shoot iron sights.