I consider TAS as TOS's continuance, thus, its fourth season - only animated, if you will
.
Anyway, I personally will go with DS9, followed closely by TNG. DS9 is the best modern Trek. Although more militarised in nature than TOS, it adresses war as a bad thing, something to be avoided, but when its due, it better have some positive results to the overal galactic in this case history. That is best adressed in The Pale Moonlight, where Sisko "hires" Garak to help him initiate a war between the Romulans, the Klingons and the Dominion. It adresses our times, most effectively, and almost exactl;y the way TOS used their universe as a metaphor for their times.
Both TOS and DS9 deliver a richly textured surrounding universe that enhanced the drama and allowed for storytelling that can touch the hearth with how human the characters were. If one watches "City on the Edge of Forever," and follows it up with "The Visitor", what will be revealed are two examples of exemplary storytelling that use Science Fiction elements to demonstrate the depth of the human heart, and the sacrifices we must make for love. The diametric opposition of these shows begins to fade... and you see the true heir appearant to the mantle of Roddenberry's dream. Good drama with strong continuity, producers who care about good characters addressing the human equation. That's what makes good Trek, and what makes DS9, to date, the best since TOS... and in some ways surpassing the original.
TNG is a good series, a strong series.. Most definetely its success lead to the rest of the spin-offs.. But its always the more "confusing" of the golden three (my phrase that characterizes TOS, DS9 and TNG). It started as a morality look on different cultures and test of non-interference with the Prime Directive in the first season, moved on to the heart of exploration with the second and third season, the fourth found finally the humanity of those characters and the rest told the stories with as much an imagination as they could. Point is, it never was consistent in the ways TOS and DS9 (even Worf's arrival in DS9 added to the rich character enviroment, without "destroying" the equation. Still, TNG is magnificent and trully, the best TV series of the late 80's and early nineties (before DS9 of course).
Voyage was a hit and miss, its 7-year run never provided in my opinion with any significant character development and Kess' death didn't even have that definitive sense of loss (you feel hurt when Kirk lets Edith Keeler die, you know Picard is saddened by Tasha Yar's death, YOU FEEL Worf's anger and hate towards Duras for both his decommension and his dear love's death, you understand Spock's pain in The Naked Time, and certainly you feel Sisko's loss over the mirror Jennifer). Just a few examples. Although it did have some fun with the holographic Doctor (IMO the only character with any depth, ironically, since he's a holographic Doctor1) and Seven Of Nine's boobies were nice(
). Otherwise, very mediocre series.
Enterprise.. I won't spend time to discuss how unfitting to this list this series is.. It feels more Stargate alternate reality than anything else.. I don't think it even comes CLOSE to TOS's sense of magic, of wonder, of humanity..
But thats all IMO..
