When I first watched in the early 70s as a six year old, it was a fun, entertaining space adventure series. From 12-15, it was full of ideas and ideals I found appealing. At university, it was a touchstone for myself and a significant portion of other students in residence, gathering every weekend in the common room to watch it, as well as the annual CBC Montreal marathon 12 hours every Labour Day weekend. TNG came along, and I watched it because I was a fan of Trek and it was new. I've watched them all (some more than others--I have liked something in each of them, but not equally). Today, I'm re-watching it with my son (occasionally my daughter, but she's more interested in the latest movies than anything earlier--my son loves any and all sci-fi that he's seen so far). I do my best to watch it without irony or excessive criticality (in the presence of my son), and the effort is often (though not always) rewarded.
I'll always give any Trek series or movie a chance (I do the same with Bond--with which I have a similar relationship of starting young and appreciating different things at each stage of life). But, so far, the best of TOS outshines the best of anything else, for me.