Something that is unguessable, uniquely yours, and also without the need to memorize or type it in seemed ideal to at least as early as 15 years ago. Biometrics seemed like the obvious choice. I got my little brother a laptop with a fingerprint scanner in 2008. Oh well.
I'm still not unconvinced (in biometrics), but I think carrying something like an electronic keychain (one that I have full control over) is more ideal today.
The problem with biometrics is that you cannot change them once they're compromised. On top of that, we leave biometric data lying around all the time - your phone is full of your fingerprints. While there might not be great systems to abuse that yet, it does make me hesitate whether biometrics are the way to go.
Biometric data not getting as much legal protection as a password can also cause issues.
I'm still not unconvinced (in biometrics), but I think carrying something like an electronic keychain (one that I have full control over) is more ideal today.