Registered is tracked at every step of the way. That with a return reciept is what law firms use with sensitive stuff where they want to have absolute proof something was received. If you're sending something valuable or sensitive, that's the way to go.
Registered mail with return receipt is probably the best option for your situation. That receipt with the address and a signature on it would be enough in court, and if they refuse to sign, the post office will not deliver.