From the manual https://www.digi.com/resources/documentation/digidocs/PDFs/90001541.pdf
DB (Cellular Signal Strength)
Reads the absolute value of the current signal strength to the cell tower in dB. If DB is blank, the XBee
Cellular Modem has not received a signal strength from the cellular component.
DB only updates when the modem is registered with the celllular tower. It is updated periodically, and
not when read.
0x71 - 0x33 (-113 dBm to -51 dBm) [read-only]
So -51 would be the best signal.