Well the standard depends on frequency range, impedance range, and type of measurement.
Some 20 years ago I obtained a near-complete set of GR 1409 standard capacitors, from a Western Electric facility that was closing.
All of them had been characterized (by whom and how, I do not know) to 6 significant digits at 100Hz, 1KHz, and 10kHz.
Totally fine for 2- or 3-terminal instruments in the audio freq range.
Over the years, as I have moved more toward 4-terminal and VNA measurement instruments, I sold the 1409's except for the 100nF one.
I have never felt the need to have inductance standards, most LCR instruments don't require them for calibration.
For 4-terminal instruments you really need 4-terminal standards, characterized as high in freq as you need.
When using a VNA, I find I don't really need L or C standards. Having a short, an open, and a load R that has flat impedance at a value near the range you'll be measuring, works fine. The calibration of the phase response of the VNA then determines its accuracy measuring reactances. For that you need phase standards or generators. The HP 8904A with opt 002 works well below 600kHz. For 1MHz, the HP 16344A Phase Standard.