+2. Registered Mail when OVERNIGHT is not mandatory. It may not be the FASTEST, however, it is the most secure.
I'd personally use FedEx overnight if you absolutely, positively have to have it delivered OVERNIGHT. You stated "Overnight". Registered mail is available ONLY for FIRST CLASS MAIL, and is NOT Guaranteed OVERNIGHT.
I used UPS Express once too often. Too iffy. Had a few letters lost or mishandled in delivery. The UPS manager told me that their "Conveyor" system in their sort facilities was not designed for "Letters" but parcels, and sometimes overnight letters slip through the cracks and end up mis-sorted. Thus delayed a day or two at best, a week or more at worst. So, for overnight, I use FedEx. For "Security", Registered Mail.
Good luck.