I watched TNG first and going back to the 60s was… something. Luckily I am a huge fan of Twilight Zone and season 1 of TOS is like Twilight Zone in space.
Spock vs Bones is also a great dynamic in any TV show. There’s some garbage episodes though. But that’s the case for the first few seasons of all the Star Trek.
Might be best to find a “Greatest Hits” list and jump around to see if you like it first. The best episodes (Balance of Terror, the Doomsday Machine) of TOS are as good as the best in any series in my opinion.
I started watching TOS after seeing the 2009 movie and was quite, er, taken aback with the change in pace lol. I was still able to handle old Trek because I watch a lot of older TV, but I’ve tried to show certain classic episodes to my husband and the pacing frustrates him even when he’s otherwise really enjoying the story.
The pacing is pretty bad on a lot of episodes. Especially how long they like to just hold on a shot of the crew waiting for something to happen. Sometimes seems like hours and hours of this.