The Star Trek: Discovery. However, the U.S.S Excelsior and the Excelsior-class starships are the true backbone of Starfleet.
The U.S.S. Excelsior was introduced in Star Trek III: The Search For Spock, which was set in 2285. Dubbed "The Great Experiment", the Excelsior (NCC-2000) was larger and more advanced than the Enterprise and it was to be the first starship equipped with a transwarp drive. However, Montgomery Scott (James Doohan) sabotaged the Excelsior to allow Kirk and his crew to steal the Enterprise and escape to resurrect dilithium crystals. After spending the next few years in spacedock, the Excelsior finally launched on a three-year mission under the command of Captain Hikaru Sulu (George Takei) cataloguing gaseous anomalies in the Beta Quadrant. In Star Trek VI: The Undiscovered Country, set in 2293, Sulu led the Excelsior to aid Kirk's Enterprise in exposing a conspiracy to prevent the peace treaty between the Klingons and the Federation.
Originally, Star Trek producer Harve Bennett intended for Kirk and his crew to take command of the Excelsior. The Star Trek: The Next Generation, the Excelsior-class starship quietly took on a greater significance as the most important starship class in Starfleet.
The Excelsior-class starship debuted in the 23rd-century but, remarkably, they are still a key part of Starfleet in the 24th century. Indeed, Excelsior-class starships fought in Star Trek's major wars such as against the Borg in the Battle of Wolf 359 in 2366-2367 and the Dominion War of 2373-2375. This means that, even though the idea of transwarp drive was scrapped long ago, the Excelsior-class ships have been active in Starfleet for almost an entire century. Further, Starfleet irals and flag officers typically use Excelsior-class ships as their personal flagships.
Along with the U.S.S. Excelsior - a newer version with the registry NCC-21445 is active in the 24th century - other famous Excelsior-class starships are the U.S.S. Hood and the U.S.S. Potemkin (Commander Will Riker served on both ships prior to the Enterprise-D), the U.S.S. Crazy Horse, the U.S.S. Gorkon, and... the U.S.S. Enterprise-B. As seen in Star Trek Generations, Captain Kirk was presumed killed by the Nexus on the maiden voyage of the Enterprise-B. Although the Enterprise-B's tenure under Captain John Harriman (Alan Ruck) was relatively undistinguished compared to Archer, Kirk, and Picard's vessels, becoming part of the Enterprise's legacy enhanced the Excelsior-class' status within Starfleet.
The Excelsior-class' sleek design, with its curved warp nacelles, is also the forebearer of the Galaxy-class starships like the U.S.S. Enterprise-D. And, although the Excelsior-class starships were never featured as the signature vessel of a Star Trek series (Sulu is the most famous Captain of the Excelsior), the Excelsior has proven itself to be the most important starship class of the franchise. After all, despite its enduring legacy and status as the flagship of the Federation, there is no Enterprise-class of starships in Star Trek.