Star Trek: Every Starfleet Starship Class Ranked Worst To Best
19. Prometheus
The Prometheus class vessel was, like the Defiant class, purely designed for combat. It was a very different project in that they could separate into three distinct component vessels (called multi-vector assault mode) to switch a one-on-one battle to a group attack. Unfortunately, the prototype vessel was overrun by Romulans, captured in an attempt to bring it back to the Empire.
While this would qualify as an act of war, full scale hostilities were put on the back burner thanks to the Dominion. Despite the obvious tactical advantages of the vessel, for some reason it did not enter full service during the war. It had however been wrapped into the fleet by the time that the USS Voyager returned home.
With the lowering of the threat posed by the Dominion, Starfleet's immediate need for warships lessened, so the Prometheus, while a big step forward in terms of design, never truly got the recognition it deserved.