I'll try to make this concise (yes, I know some here will say that would be a first

):
IMHO, Star Trek had been and should be foremost about exploration, the exploration of the macrocosm (outer space) and the microcosm (i.e. the human condition) to qualify as "real" or "true" Star Trek according to Gene Roddenberry.
As strange as that may sound upon re-watching TAS, once you blend out the presentation and its target audience, there is so much "Star Trek" under its skin, that often it seemed to me, that TOS was a compromise Roddenberry had to accept but TAS gave him plenty of opportunities to present what he had tried to feature originally.
In a manner of speaking it feels to me like TNG is the continuation of TAS but for an adult target audience.
The biggest flaw of TMP, up to this day, is the deletion of the "Spock cries" scene. Its producers misunderstood that VFX is not what makes a movie great but the characters. But as a genuine science fiction film this one is among my top three with
2001 and
Mission to Mars.
After that, IMO, Star Trek become too derivative once it started to compete with
Star Wars (movies) or
Babylon 5 (DS9).
Having "survived" the first two seasons of VOY I thought the show was really getting much better and truly went "where no man had gone before". Could be that their isolation from home and really being on their own gave it a genuine Star Trek feel I never quite had when watching TOS or TNG.
Bob