The lower the better, its hard to find the truth of what the IR actually is. And a lot of the companies won't tell you what the actual IR is. SMC sells a device that will tell you the IR of your batteries. I use my Thunder Power TP820 charger. It's far from the best. But its better then nothing. Here is an example of 3 of the batteries that I own.
I'll list them per cell all are 2s lipos. These numbers are fresh after a full charging at 20 amps.
CRC Maxxcell 5600mah 65C. 7...5 miliohm. 12 total
BMI Motorsports 6500mah 60C. 6....5 miliohm. 11 total
SMC 7200mah 70C. 4...3 miliohm. 7 total
These numbers are probably far from the true IR but for my testing using the same exact setup. It's good enough for me.