The eleven-digit National Identification Number issued by NIMC. It never changes, and almost everything else in Nigeria is gated behind it: a SIM card, a bank account beyond the lowest tier, a passport, a driver’s licence, a pension account.
First-time enrolment is free. Correcting a record afterwards is not.
Do not confuse it with the BVN, which is a banking number issued by a different body for a different purpose. You need both.