I think, to me, it's love.

To be able to love and to be able to be loved is the most precious thing.

I get the "time" answer but honestly...time isn't precious. It is what it is regardless if we acknowledge it or not. Love, on the other hand, can actually manipulate time or at least our perception of it. It can alter how we spend our time and what we do with our time. I would agree that time is a resource except we all have the same amount of it, we all are affected by it the same way...it, in my mind, is just a constant and not a resource. Therefore I can't see it as precious. It exists for literally everything and everyone. Love...is precious because, while it does exist in everyone and probably everything...not all of us get to experience it. I truly feel for the ones that never get to.