The full answer is long and complicated but the short answer is almost surely 1) the proper air pressure at zero cost (meaning, infinte zero slope in terms of price/kg); followed by 2) latex tubes (at a price premium over butyl tubes of just a few dollars; and 3) the proper tires.
The short answer is that rolling resistance can be translated into an "equivalent mass," and since rolling resistance applies to the entirety of mass combining both rider and bike, even a small improvement in the coefficient of rolling resistance multiplied by total mass produces an "equivalent mass" reduction greater than a direct reduction in mass.
The attached figure gives an example of how even small changes in coefficient of rolling resistance can have large overall effects, and is the basis for the aphorism, "if you're a weight weenie you should definitely also be a Crr weenie." For a fuller response (though still not completely full response, which requires some mathematical reasoning), you can watch this Youtube video:
