What is the name of the first Defiant-class starship?

The first Defiant-class starship was the USS Defiant.

The ship, registry NX-74205, was developed as a heavily armed prototype intended to counter the Borg. Its compact design, ablative armor, powerful weapons and borrowed Romulan cloaking device made it very different from the exploration-oriented vessels usually associated with Starfleet.

Commander Benjamin Sisko received the Defiant in 2370, and it became closely associated with Deep Space Nine and missions into the Gamma Quadrant. It first appeared in the third-season premiere of Star Trek: Deep Space Nine, “The Search,” and later fought in the Dominion War and appeared in Star Trek: First Contact.

A common mix-up is USS São Paulo. After the original Defiant was destroyed in 2375, Starfleet gave Sisko that replacement ship and permitted him to rename it USS Defiant. São Paulo was therefore the successor, not the first vessel of the class.