I will attempt to analyze the multiple series from a hard/soft sci fi perspective.
Enterprise is probably the hardest. Very RPG-like how they operate to their self made rules in what compared to later series is extreme limitations. Life is a lot different when the fastest transport ship flys at warp 1 and everything isn't deus ex machina away with AI or technobabble. On the other hand they overdo the "wooden ships and iron men" thing a little and there is too much "pew pew" action.
Next up is DS9. Some technobabble for dramatic license but still reads like a paper and pencil RPG fiction (which is good). Some "crewman of the week" cheesy soft filler episodes but mostly hard science fiction story. Has a reasonably hard science overall story arc. Arguably the best overall series. Has the best "hard" written "soft" character in all of trek, the Cardassian "tailor". He's not really a tailor, hope this isn't too much of a spoiler. Best sci fi on TV, maybe.
TOS is all over the map. Sometimes they did a good job of both hard and soft sci fi at the same time. Then there's "space hippies" or "spocks brain" or "spock needs to get laid" which we'll pretend never happened. There is an unrealistic (by hard sci fi standards) fixation on alien planets being the wild west desert. You NEED to watch certain individual episodes, like the doomsday machine.
TNG is the softest series, its General Hospital soap opera, on a space ship. Toward the middle either its a short story arc, sometimes halfway decent, or "crewmember of the week special story". The Wesley character is kind of the series Jar-Jar and you'd do best to skip episodes where he is "crewman of the month". Also the romance episodes read like fanfic from /r9k/. The alien civilizations are bad caricatures.
Generally across all the series from a hard sci fi viewpoint if an episode involves space nazis, time travel, Ferengi, romance, or a holodeck accident, it can be ignored with little to no loss with very few exceptions.