I'm a huge nerd so I think about this ever few weeks. Onward!
TOS
DS9
TNG
VOY
ENT
TOS is 1st. It was and still is, revolutionary. Outside of production that from the 1966 TV, and various other social/cultural oddities from that time, it's still brilliant storytelling. A lot of what makes Star Trek special started here. Like most people, you love what came first or at least, what you saw first. I grew up watching Star Trek on Sundays on CBC reruns in the 70s. There were few shows that burned into your mind like this one. People who start with TNG feel this, and I understand it. TNG after all is made in the image of the original.
DS9 is 2nd. Probably the best Star Trek overall. The best characters, the best emotional impact and the best writing. The writing really does holds up, and while the show is showing some age, it's aged far far better than all the others because of the writing.. I still hold it up as one of the best SCI FI shows of all time. I could go on about characters, story, acting, writing, scope, until you'd be sick of reading.
TNG is 3rd, and it's still great. While it's episodic nature actually made it easily watchable when on the air, it's lack of overall arcs and connected tissue seems to have been a missed opportunity in retrospect. Some episodes are timeless classics. The first two seasons and the last ..are rough. Best final episode in Trek history however.
VOY is 4th, and I'm still mixed about it. Hands down the best concept in Trek. And the fact that in many cases the concept was blown. There was a great cast there, that seemed to be ignored after a few years. While DS9 went for a serialized storytelling format, Voyager went episodic again. My theory for this is that at the time, DS9 was disliked. Paramount didn't like it, and wanted just more TNG. Instead they created Voyager for the new network PSN, and ignored DS9. This helped DS9 as they did whatever the F they wanted, and Voyager had to be "as popular as TNG". While it was still worth watching and they reached some highs.. the lows were too much and overwhelmed the show.
ENT is 5th and is a show that also seems a little doomed from inception. If you were a writer and was given a tv show, told it was sci fi, you'd probably feel pretty good. Pure creativity, go out and push the limits. Wait. It's Star Trek. Okay.. so you have to play in that sandbox with establish universe. No probably I can do it. Wait. It's Star Trek, and it's set before all the other tv shows and movies. Well, fuck. It had moments, it had some interesting stories, but it never had a chance. You can imagine how I feel about Star Trek: Discovery's chances.