What can I do to make sure someone gets a Letter?

What can I do to make sure someone gets a Letter through the USPS?

Is there a way for me to send a letter to someone without any possibility that they won't receive it or a wrong person gets the letter?

Registered mail, signature required.  
Whether USPS fails, that's pretty random.

Is certified the same as registered?
Having to sign for it is a fairly sure way.

I sometimes think priority mail is safer but probably just because it costs more....at least it would be noticable if tampered with.

I think there are probably other ways depending on how much you are willing to spend but they will all involve other people or machines...nothing is risk free unfortunately.

In theory, registered mail with signature works. But the signature may not necessarily be the person it's addressed to- someone else may sign for it. There used to be something called "restricted delivery" where the mail carrier needed to check the ID of the person against the name, but I don't know if that is still offered by USPS. (Unsurprisingly, the USPS site is about as clear as mud and I can't tell. https://faq.usps.com/s/article/What-is-Restricted-Delivery )
I think people can refuse delivery, also.
But unless you're going to have a hand delivery, warrant-served style, I think it's the best option you have.