I love TNG, don't get me wrong.. I think it's fantastic.
But, I prefer TOS for a couple of reasons.
1) I prefer the TOS crew. I prefer Kirk because I can relate to him. That doesn't mean Picard is not a badass in his own right, but Kirk is more like me as a person.
2) Spock is a fantastic character as is Data, but nothing beats the Kirk/Spock dynamic duo.
3) TNG tried to replicate McCoy with Dr. Pulaski but that didn't really work very well. She was a good character, and offered a counter for Picard, but, not a good companion. They wanted to so desperately avoid a romantic relationship with Crusher and Picard that the constant tension between them seemed odd. By contrast, McCoy, when added to Kirk and Spock is the perfect group of 3 guys all very different all with their own dimension to add to the group.
4) TOS is so fucking Cold War old-school it's ridiculous.. There's an episode about two planets waging endless virtual computerized war, where they simulate detonating thermonuclear bombs over their cities and based on the simulated results, they kill people who were killed by some probabilistic function. It's insane.
Long story short, Enterprise shows up with some loud mouth diplomat who shirks Kirk's authority and forces them to enter this warzone. Upon beaming down and being politely greeted, they're immediately taken into custody.. The guy asks him straight up:
"You mean.. we are to be killed?" The cold-hearted response in the affirmative is just as great.
Now, TNG had some great characters that TOS didn't have any semblance of like Riker, Troi, and Warf. Those characters' best episodes are typically among the best of the series.
Beyond the main cast, I thought the stories in TOS were very well done, often more so than TNG. TOS Seasons 1&2 are fantastic and the quality of writing is often superior to what you would find in TNG over the same run of episodes.
Now, TNG has the absolute best Star Trek episodes to date, including Best of Both Worlds, etc, but, I don't think that really speaks to the average episode and if we remove TOS Season 3, then I think the Original Series is marginally better and more enjoyable.