What wire is best for hot tub wiring?

The best wire for a hot tub is 6 AWG copper THWN-2 pulled through conduit, on a 50 amp GFCI breaker. That serves the standard 40 to 60 amp spa with the wet-location rating the outdoor run requires.

Romex is the wrong tool here: the outdoor and underground portions are wet locations, and NM cable is dry-only. Most manufacturers also specify copper conductors exclusively.

The four-wire circuit, two hots, neutral, ground, feeds a disconnect within sight of the tub, at least 5 feet away, then continues to the spa panel. GFCI protection is mandatory, per NEC 680.

Check your spa's manual for its exact minimum circuit ampacity: 6/50 covers most, but big swim spas want more.

Verify the run with the 50 amp wire size calculator, including the length adjustment for long backyard pulls.