I have to pick Retinal Scan. Being someone who has her fingerprint scanned dozens of times daily, and does so for others who come to cash payroll/government checks, fingerprint analysis can be flawed. Those people who have chronically dry skin, like older people, have difficulty having their fingerprints taken. And those people who work in hands-on fields, such as construction, mechanical, etc, take damage to their fingers which will actually alter their prints, making them unusable.

I suppose, as an after-thought, that there are potential ways for a Retinal Scan to fail as well...

Can I change my vote to Numeric Code?