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:
- 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)
- 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.
- 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.
- 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.
- 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.
- 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.
Star Dreck Discard makes Picard space Jesus then he dies and comes back as an android Picard...who is dying of old age. STD and Discard don't deal in logic and absolutes but "destiny" and "chosen one" crap. Plus weird Klingon sex scenes trying to be game of thrones and F bombs.
Which is funny because I've seen people bring this up in defense of QT doing an R-rated Star Trek movie ("STDiscovery has delved into R-rated content, so why not?"), which makes no goddamn sense whatsoever, and I say this as somebody who loves gritty films. In fact, STDiscovery actually hurts their argument because many longtime, dedicated fans don't like STDiscovery, saying it deviates too much from the lore and themes of Trek for that very reason. Star Trek isn't supposed to be about the "horrors of space", so a Klingon rape scene just feels jarring and out of place. It felt like it was thrown in purely for shock value.
My favorite series is probably The Next Generation, followed by Deep Space Nine.