Possibly the only
Federation starship class designed specifically for combat, not exploration.
The four
Defiant Class Starships are:
- NX-74205 - USS Defiant: Prototype and class ship; stationed at Deep Space Nine; destroyed by the Breen in an attempt to recapture Chin'Toka system (The Changing Face of Evil). Commanded by Captain Benjamin Sisko.
- NCC-74210 - USS Valiant: Destroyed by an unknown class of Jem Hadar vessel (Valiant). Commanded by Cadet Tim Watters after the original captain and all senior officers died.
- NCC-75433 - USS Defiant: Originally USS Sao Paulo; Recommissioned as USS Defiant after the destruction the original USS Defiant (NX-74205). Commanded by Captain Benjamin Sisko, and later Commander Kira Nerys.
- Unknown: Never seen or mentioned on the show, to the best of my knowledge.
Note: In an early episode of Deep Space Nine, it was stated that The USS Defiant was a Valiant Class Starship but since then it has been named Defiant Class numerous times.
Some information taken from the Starfleet Ship Registry Database: http://www.webzone.net/rowan/sb907/ssrd/