What is the most secure 2FA method?

[jonah (Jonah Aragon)]

@anonymous210384

For example, a platform authenticator integrated into a mobile device

That definition is clearly not describing a Yubikey. You said:

@anonymous210384

Yubikey is not a roaming authenticator.

There is no definition where this could ever be true, because a Yubikey is not a client device (see spec for definition of client device).

a roaming authenticator (e.g., a USB security key fob)

Web Authentication: An API for accessing Public Key Credentials - Level 2

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@anonymous210384

No, you knowing something via memorization is not the requirement.

You are incorrect. multi-factor authentication - Glossary | CSRC