Short answer: for a beginner-friendly SaaS login in the US or EU, SMS OTP is the simplest managed starting point; authenticator apps are stronger, while an email code is a fallback you must build and evidence yourself. The decision is less about a clever factor comparison than about what your compliance file can prove six months later. A password-reset message with a short expiry is a useful test case: the user needs reach, the security team needs an audit trail, and the storage owner needs a...