The kind that Google and Microsoft (and maybe others) do is pretty great. You get a push notification on your phone and just tap "Yes", and you're in. Of course

The kind that Google and Microsoft (and maybe others) do is pretty great. You get a push notification on your phone and just tap "Yes", and you're in. Of course, you should still have a fallback like OTP for when your phone is not internet-connected, but it feels really seamless and is a very minor inconvenience.