If you're fully entrenched in the Apple ecosystem, Keychain is the most seamless option. I don't use its "strong password" recommendation system, however, since

If you're fully entrenched in the Apple ecosystem, Keychain is the most seamless option. I don't use its "strong password" recommendation system, however, since it breaks on half the signup flows I try it on, sometimes botching things so badly I have to use the "forgot password" link to log in the very first time.