Deep Space Nine for sure. The most interesting characters, setting, chemistry between characters, most of my favorite episodes from the entire franchise come from DS9. It was daring, it was bold. I know that by today's standards, it is tame. But back then, it was something new allright.
TNG is a close second, and the show I started watching first. But it's a bit to polished and perfect for me to be as good as DS9. Voyager and Enterprise are highly enjoyable, and I think I prefer Enterprise, but that depends on my mind and which season. At times, Voyager shone like a bright Star Trek beacon. And then, sometimes, it was shittier than a New York sewer.
TOS..... I love TOS. It gave us the beginning. But 60's tv just isn't my thing. A bit too camp. Certain episodes I can always watch, no problem. And the characters are great. But as overall enjoyable tv.....no.
And Discovery. That show is really doing it for me right now, but we're only half a season in. It's a sign of the times that people are hooked way too fast on shows these days and claim that 'this is DA BEST SHOW EVAH!!' only to drop it when a new show arrives. And that's unrealistic. A good tv show needs some time to develop. So, I will reserve judgement untill season 1 is over. I do really dig it sofar.