Its been qite a while since I put an NIBP machine on test, but I always considered a mercy sphyg the most accurate comparison for pressure.
The NIBP test devices from companies like Ultramedic and Rigel should be more than sufficient for doing planned maintenance inspections. (So long as they are calibrated)
If you are doing research - then a mercury column (mmHg) would my 'go to'.
After all - that is the measurement (mmHg) you are trying to check.