At this point, I think I've rewatched the first season of TOS more than any other season from any Star Trek series. It's like an endless spring for my brain to drink from.
The older I get the more I appreciate TOS, and it has always been my favorite. The bolded is pretty much the main reason why, there was so much creativity in play that much of the later shows are totally devoid of.
The original Star Trek tickled my brain in a way no other Trek show has come close to.