TOS 1 (based on what STEPhon IT said about world building), if not TNG 3 (which also solidifies the best that TNG 1 and 2 did but with the extra flair needed... Along with Michael Piller, Ira Stevhen Behr pretty much defined the style and content and, indeed, the soul of 80s/90s Trek that's cherished up to this day.)
I'm not into the new series so these are my picks as well with the top three being TOS,TNG and DS9. If I had to pick the best then TOS Season One gets the nod by virtue of being first. They laid the groundwork for everything that came after and it still holds up 50 plus years later. DS9 Season 4 would be a very close second.
More of my favorite episodes are in TOS S2, but the mean level of TOS S1 is higher and S1 still has some great episodes. I guess if it's the overall season, TOS S1 wins. TNG S3 deserves honorable mention.
My vote goes to TNG season 3, it was when TNG truly became the show it's remembered as, there are very few episodes I don't care for with the vast majority being good to great.
If we're doing one per series: TOS - 1 but I prefer 2 TNG - 4 (yes, I like it better than 3) DS9 - it's tough to choose between 4 and 6 VOY - 4 ENT - 4 There aren't enough seasons of DSC or PIC to make a meaningful choice. Of other note: There seems to be something about fourth seasons during the Berman Era.
My favourites from each series: TOS - Season One TNG - Season Three DS9 - Season Five VOY - Season Four ENT- Season Three DIS - Season Two PIC - Season One Was it Scotty's season three hair that swung it?
Fortunately it's not absolutely necessary to choose only one of them.... ....but the one with 'Who Watches The Watchers' might get picked, that's a legendary episode for me. However, then there's the thing with 'The Best of Both Worlds', one episode on both seasons. In some way it's possible to watch only one part and get away with it but.. it's just not the same. Impossible question. Seasons 3 & 4 are connected so strongly.
I’m gonna go Voyager 4 then DS9 2 - I love TNG, but really love it more once Guinan shows up in season 2.
My top five: DS9 season 6 The Dominion War arc, Far Beyond the Stars, The Magnificent Ferengi and In the Pale Moonlight - some of the best Trek episodes. It falls off a cliff towards the end, but the good far outweighs the bad. TOS season 2 Amok Time, Mirror Mirror, The Trouble with Tribbles, The Ultimate Computer, Journey to Babel Discovery season 1 The mirror arc, Context is for Kings and Magic to Make the Sanest Man Go Mad all blew me away. Voyager season 5 Two of the most underrated episodes - Counterpoint and Someone to Watch Over Me - plus great stuff like Drone, Dark Frontier, Relativity and Equinox Enterprise season 4 Loved the augment, mirror and Terra Prime storylines
I'm gonna' chose one for each (finished) series: TOS: season 1 Hard to describe - this season is basically what the whole "Star Trek" thing is about, it opened the whole universe for me. Probably also my pick for "best Trek season. period." TNG: Season 6 A bit of an outlier. Yes, season 3 & 4 are also super great, and season 6 starts and ends on weaker two-parters. But I just love most of the episodes in this season so much, it's just overall great quality. DS9: season 6 I was never a fan of how the Dominion War was depicted as essentially WW2 in Space. But IMO this season is the very highpoint of the Dominion conflict, and is basically what DS9 as a whole is all about, with a great arc in the beginning, and most episodes being related to the main story. VOY: Season 4 IMO one of the best Trek sesons, period. So many great two-parters, single episodes, and the arrival of Seven of Nine really changes things up, and makes her & Janeway one of Trek's great duos. ENT: season 3 By far the best season-arc in Trek. There's some clunkers in the beginning, but this is IMO how modern Trek should be like - there's an overarching (very Trekkian!) main plot, big mysteries, a new enemy that is thoroughly explored, and lots of great character arcs for all main characters that are organically interwoven with the main story. And all while still keeping the "story-of-the-week"-approach of Trek.
I think the single best season is the original series season 2 and second best is the original series season 1. For third place, tie between TNG Seasons 3 and 4.
I mean no disrespect...because it's all subjective of course...but are you TNG Season 7 is one of the best? I recently did a rating of every episode from every season (another thread on here somewhere) and S7 was really abysmal.