It is a hard one because the only computing/memory device you have with you at all times, requires no batteries and not connected to any networks (yet) and not vulnerable to probing/observation (yet) is your brain! But memory is too unreliable unless everyone trains for it.
Crypto keys are great but you can lose them and once shared they are keys to you kingdom.
Specific security devices are great but you need to remember to have them with you. They can get lost or broken so you need backups.
Google authentication is convenient but they can ban you. It is also a 3rd party to trust.
Passwords suck but might be the best of the worst. Advantages: password managers can be used to make password useless for other sites and people conceptually understand it.
