Where is the first Boeing 747-8 ever built stored at today?
The first 747-8 built was not a protype, but a production unit. The first delivered aircraft in both the versions (747-8i Intercontinenal and 747-8F Freighter) are in service, the Passenger version with Lufthansa and the cargo version with Cargolux.

The first 747-8 customer was Cargolux for the 747-8F version.

The first 747 ever built, 747 RA001 'City of Everett' is in Museum of Flight in Seattle.