Using Analytics to Track Registrations

What is the best way for analytics tracking after a user has successfully registered?

One option would be to use Events & Webhooks. Depeneding on what you want to track, you may be interested in the following events:
-[user.registration.create]
-[user.registration.complete]
-[user.registration.verified]

You could also enable the email verification gate. Then you could call your tracking event on this themed page which would only be fired when the user exits the Email Verification gate as the result of completing their email verification during registration.

Another option may be to add an UTM code to the redirect_uri used for self-service registration and consume that in your application as the result of a user completing registration.