I grew up on TOS... and it has a special spot in sci-fi for me. I can still enjoy episodes from time to time, even though I've seen almost every episode a dozen times or more (the only ones I haven't seen much are those I can't stand, like Turnabout Intruder, The Children Shall Lead or Cat's Paw). But... despite it aging well, it is OLD. And limited. So I have to start with TNG.
1. TNG - this was the series that "saved" Star Trek. Not only that, it features Patrick Stewart, who I still feel has been the best Starfleet captain ever to appear in the franchise. While I can't stand Season 1, and Season 2 has only a few redeeming episodes, the 5 seasons to follow are all terrific with only a few "lemon" episodes here and there.
2. TOS - It's 2nd, only because TNG displaces it. The early Captain Kirk with Spock and McCoy was such a phenomenal team. There is much to appreciate in Shatner's Kirk. Once you get to the point where you're so familiar with all the episodes, you don't have to pay attention much to the main plot and instead focus on all the little interpersonal details. And there's quite a bit to it. Sure, it has its flaws and its dated... but this, this is the wellspring that started it all.
3. VOY - I know a lot of people don't care much for Voyager relative to the other ST series, but this was the last of the prime universe series and it had a very good run of 7 seasons. I didn't care much for season 1 and I just couldn't wait for them to get rid of the Kazon. But Mulgrew "grew" into her role and became a very likable, believable captain. Robert Picardo was stellar as the EMH. I think he really made VOY. The supporting cast was generally OK with moments of really shining. But VOY got to explore so much that TNG did not.
4. ENT - I had high hopes for this one. Didn't like the first 2 seasons. I really think seasons 2 and 3 should've been "merged" and the "fluff" taken out. It moved too slow and lost viewers. Season 3 really started to hit stride, but by then the writing was already on the wall. I felt a little mixed about this "pre-universe", but there were so many great episodes created. And the crowning achievement, "In a Mirror, Darkly"... It was cancelled before its time. It should have had a 7 season run. Definitely.
5. DS9 - I had to put this last, just a hair behind ENT, despite it having so many redeeming qualities. DS9 explores a lot of territory that the other ST series didn't get to do. I also think Benjamin Sisko was probably the 3rd best captain of all. The only thing was just the soap-opera like quality this series fell into from time to time. I guess it was inevitable given the context. But the last few seasons did a great job of breaking out of that. The Dominion War was an epic story arc. This is an important series in the Star Trek canon and I'm glad it exists.
X. DSC - I've only seen the first few episodes because I don't have a CBS subscription. I'm not very keen on it and I absolutely loathe the "spore drive" idea. Still... I'm looking forward to one day watching it. Until then, I can't really rank it.