My bad; I've added it as an option. Don't know how I managed to forget that one.

I did a full rewatch of all of the older series from late last year till June this year, so this one is easy for me. My personal ranking: 

1. Voyager- Unpopular opinion I know, but I've always liked Voyager best, ever since I was a kid. Great cast of characters (there isn't a single main character that I don't like, though Kes is probably the weakest of the bunch, trading her in for 7 of 9 was the best decision they could have made), interesting setting, love the heavy focus on the Borg, best set designs, best uniform design of all the series (DS9 only used it for the first 2 seasons before they switched to the TNG movie uniforms)
2. DS9- I did not like DS9 much at all when I was younger, but as an adult it speaks to me so much more now. Alot of the characters that I didn't like originally have grown on me over time, like Bashir and Quark. Love the heavy focus on religion/faith vs science, and the focus on war, neither of which is seen much in other Trek series.
3. TNG- Most people's favorite I suspect, I personally find it a bit overrated though. First 2 seasons were tough to get through on my recent rewatch, there was too much TOS influence in those 2 seasons which really dragged it down. Gets better on the later seasons due to less influence from Roddenberry. I like most of the TNG characters, but TNG imo has the 2 worst Doctors of all the series, Crusher and Polaski, one of which was insufferably boring while the other was a poor copy of McCoy.
4. Enterprise- Very underrated show. First 2 seasons were a bit hit and miss, but the show got way better on 3rd season with the Xindi arc and stayed good on season 4, but sadly the first 2 seasons had already done their damage and caused the show to bleed too many viewers, so the show was cancelled before it got the full 7 seasons it deserved. Mostly good characters, not a big fan of Phlox though (3rd worst Doctor after Crusher and Polaski), and Hoshi and Travis were hit and miss at times, but I liked the rest of them.
5. TOS- By the time I was born TOS was already quite old and dated, and it has only got older and more dated since then. I was just never able to get into it, everything from the design, to the effects, to the acting feels corny and dated to me. I like most of the TOS movies, aside from the 1st one, with Star Trek 4 being my overall favorite Star Trek movie (followed closely by First Contact), but the TOS show just isn't for me. 
6. Discovery- Saw one episode of it and absolutely hated it. Visual design is horribly inconsistent with the other series, characters are terrible, plots are terrible. CBS locking it (and Picard) behind a paywall in the US also leaves a very sour taste in my mouth.

Haven't watched Picard yet, not sure if I ever will to be honest, as I have trouble believing that the same CBS that made Discovery actually was able to make a worthy successor to TNG era Trek. The writers on The Orville seem to understand TNG era Trek fans better than CBS and their writers do. 

Deep Space Nine.

-Incredible individual episodes? Let's see, In the Pale Moonlight, The Visitor, Duet. Check.

-Fantastic cast? Arguably the best from top to bottom of the franchise, including easily the deepest well of non-cast recuring characters (Garek, Rom, Nog, Gul Dukat, Kai Winn, Martok, and Weyoun just to name a few).

-Worthy captain? He might not be as iconic as Kirk and Picard, but Sisko carved his own place in Star Trek history as a more complex character and the man who the Federation needed. The hints of what he'd become can be seen as early as the pilot episode when he uses Nog's freedom as a bargaining chip to keep Quark on the station.

-World building. OMG, yes. We have a world rebuilding after a brutal occupation that has located their gods. We have the dying totalitarian regime trying push off its collapse and justifying its existence. We have an ascendant totalitarian regime. We have, arguably, the most movement in defining the Klingons (impressive as TNG did a pretty solid job itself with them). And we have the Federation, fat and happy through TNG, having to face a civilization-altering threat. Yeah, the Borg were also a threat, but that happened over a weekend. The Dominion shook the Federation out of their Roddenberry complacency and forced them to really grow up in a dangerous galaxy.

-Character development. The most successful Trek at letting their characters grow as the series went along. It wasn't just setting things up to give each character a role, they all followed it to its conclusion. Again, this is true even of the supporting characters. I mean, is there anyone on Voyager with the depth and complexity of Garek or Dukat? Did anyone on TNG grow as much as Nog?

-Overall story. Easily the most successful long-term storytelling that the series has ever attempted. This show built and built so that the series got better each year for its first six seasons. Loyalties change, enemies become friends, and then enemies again. The Dominion storyline was a slow boil, introduced to the series by the machinations of Grand Negus Zek of all characters in a 2nd season episode mostly about a female Ferangi not being allowed to do business (a plot line that, itself, would continue all the way through the show's final episode, though truthfully, not its strongest thread) and wouldn't fully begin until the end of the 5th season. And there was so much going on that it never felt like the cold war buildup was taking too long.

Yeah, there have been other great Trek series. TOS and TNG had amazing individual episodes and a few great characters. But, DS9 put it all together and built what I would rank as one of the top television shows of the 90s.  Sadly, Voyager starting so soon after TNG left the airwaves meant that it was treated as a bit of an also-ran.