Why couldn't there be dozens of "best" ships in a fleet that no doubt features hundreds and later is indicated to have thousands if not tens of thousands of ships? It doesn't sound like an exceptional feat of manufacturing - especially when we see even more of the Nebula class ships of the same size and basic design features. (That is, stock footage of ships of that class gets assigned more names backstage and in dialogue than the very few assigned to Galaxy appearances; make of that what you will.)Yet, we see dozens of Galaxy-class ships during DS9, which suggests that either the Federation's ship-yards are much more efficient, or the Galaxy-class is actually considerably less advanced than other starship classes built later.
Even in TMP, we don't really learn much about the state of the art or the total strength of the fleet or the manufacturing capabilities of the Federation. Kirk doesn't obsess about the renovated Enterprise because she would be the best - he obsesses exclusivel because she is the Enterprise. There might indeed be scores of more advanced ships out there at the time, and Kirk has no interest in dragging his butt from behind his comfortable desk for those.
For all we know, Kirk in TMP loved his desk job. But possibly he instead calculated that he needed to take some job to stay current in Starfleet while he was waiting for the Enterprise to return to active service, and he had no interest in going aboard other ships so the prestigious desk was where he settled with pleasure.
Timo Saloniemi