TOS has the best tone/feel of all the Trek series’, but I’m not keen on the Technicolor sets/costumes, the episodic storytelling, or the steadfast focus on Kirk/Spock/McCoy. TNG & especially DS9 did better jobs utilizing and developing their characters.
Just about every series was striving for vivid color once the networks began regular color broadcasting in 1966, and NBC parent RCA wanted to sell a lot of shiny new color tv’s. Developing the supporting cast simply wasn’t a thing in 1960’s tv. We’re lucky the Enterprise even had the impression of a constant crew.