50 NPLC will give the lowest noise reading.
In this case each measurement is integrated over 50 power line cycles, which essentially means the measurement is continuously averaged for about 830 ms (for 60 Hz power). You can further include the digital filter to average over many of these measurements.
Note that you can easily configure these settings to have a measurement take over 10 seconds before you see any kind of data.
Let me know if I need to clarify anything.