Hands down, definitely TOS. It was fun while also being intelligent. They weren't lazy with the alien design and actually had some alien looking aliens. There was little to no technobabble and most importantly it was character driven. Kirk, Spock and McCoy are what make that show. They were fun characters that were relatable.
Secondly, Kirk has never been topped. He was charismatic and intelligent. Always put others before himself. If there was a dangerous situation he faced it head on and never hid on the bridge. He was charming while also dealing with the weight of command and the loneliness that comes with it.
In contrast, TNG while still entertaining was weighed down heavily by Starfleet regulation. Now the captain was stuck on the bridge while the first officer handled the away missions. Everyone treated everyone else with a level of professionalism, even off duty. They'd rarely let their hair down so to speak. TNG was excellent but in a different than TOS.
Ds9 in comparison was a breath of fresh air after TNG. I hadn't realized how stuffy TNG had gotten until DS9 kicked the door open again. It got back to being about the dynamic characters and their interactions with each other. While there was technobabble to be thrown about here and there it didn't drive the plot, the characters did. It's my second favorite series after TOS.
Voyager by contrast.....eeeee. The first couple seasons were just terrible. It was the random abstract sciencey obstacle of the week that was padded until they'd come up with the deus ex machina technobabble solution of the week. That would then be conveniently forgotten about when it could have been helpful in later episodes. After that they had a couple good seasons that really developed the characters and their struggles before slipping back into the dreck and ended on a real dud note.
Enterprise was never terrible. The first season really developed the beginnings of Starfleet's journey and started to lay the foundations for the development of the Federation. Season two....I think season two is what really cemented the show's fate. It was pretty dull and I think a lot of people gave up after that. I know I did. Which was a shame because season three was great. Season 4 was the best. It's a shame they kept all these great ideas on the backburner instead of interspersing them through out the previous seasons. Had they done that the show might have found its legs a lot earlier on. But instead they horded all of the good stuff and then had to rush through it all in their last season. The idea for the finale was interesting but I think it really robbed them of their last chance to shine and stand on their own two feet.
