Adding new service

I am putting in a 50A-2pole breaker to run a 220 line in the service box. I need to run the wire 45 feet through an un- finished basement. It will connect to a Spa panel on an out side wall. The panel is 125A with a 50A GFCI breaker. The hot tub is 120/240 15.5/40.5. My question is what kind of wire do I need to use for the 45 foot run from the service panel?