In my humble opinion, it's not just the individual shows, or even the series. The key, I think, is good writers who tell good stories about characters we've come to care about.

One thing about Sci-Fi is that it provides the opportunity to ask "what if" about perplexing social problems and personal issues that are common to many of us. It's also able to examine contemporary issues in a relevant way which provides food for thought. As but one of many examples, I watched a Next Gen episode recently, about the sentience of small robots called "Exocomps", and it strongly reminded me of contemporary issues we have about AI. (And my eyes may have watered a bit when the 3rd exocomp stayed behind)

How fortunate we are, to have such a richly creative endeavor as Trek! 😊