I've set up an impedance analyzer (Hioki IM3570) to perform a frequency sweep from 100 Hz to 1 MHz, displaying impedance magnitude (|Z|) in green and ESR (Rs) in yelllow. The first sweep is of a precision .02 ohm resistor; the top of the screen is at .1 ohms and the bottom at .001 ohms:

Both |Z| and ESR are right on .02 ohms until at the high frequency end of the sweep |Z| rises do to the inductance of the resistor leads.

Here's a sweep of a really good polypropylene 5 uF capacitor, using the ordinary settings for the analyzer; the top of the screen is changed to 1000 ohms. Notice that the ESR curve at the low frequency end of the sweep is noisy; the ratio of |Z| to ESR is 4 orders of magnitude, making this a difficult measurement:

By using some averaging, the ESR curve is cleaned up at the low frequency end:

Here is the same capacitor, but with the top of the screen changed to .1 ohms:

Notice that there's a downward jump in the ESR curve (yellow). This is due to the range change that occurs during the sweep; the instrument needs to go in for calibration. The question is, which part of the curve is accurate, the left side before the jump or the right side. I measured this same capacitor on a Wayne-Kerr analyzer and determined that the right side is accurate.

The lowest ESR is around 3 milliohms at around 3 KHz. I get the same values on other analyzers.

I would like to mail this capacitor to you, Jay_Diddy_B. If you're interested PM me a good mailing address.

Just for reference, here's a sweep of a Polyester (MKT) capacitor. The lowest ESR is nowhere near .001 ohms. It has been my experience that Polypropylene caps have the lowest ESR, much lower than Polyester caps: