My TOS opinions:
Season 1: Strongest overall. Of course there are some clunkers, like Mudd's Women, The Alternative Factor, Operation: Annihilate!, etc. But overall it has the highest number of great episodes. It's also interesting to watch (particularly in production order) to see TOS as we came to understand it slowly unfold. Early Season 1 is sometimes unrecognizable - and not just because they hadn't figured out things yet like the Federation. it also changes structurally from week to week, with some episodes coming across as "The Adventures of Captain Kirk" while others came across as far more of an ensemble feel than the show later had, with Yeoman Rand still on the show, Uhura and Sulu getting more to do than they would later, and some recurring secondary folks like Riley. But by the end of the season it became the Kirk/Spock/McCoy show we came to expect.
Season 2: A few amazing episodes (Amok Time, The Doomsday Machine, Mirror Mirror, The Trouble With Tribbles, and A Piece of the Action). However, a lot more mediocrity than Season 1 - and less versatility as the show fell more into the "monster of the week" format. I feel like a lot of the thoughtful, high-concept elements of Season 1 - which sometimes came across as kind of the Twilight Zone with more "heart" - went away here for more straight-ahead action-adventure. Just didn't do it for me as well.
Season 3: I like Day of the Dove and All Our Yesterdays. There's a handful of other episodes (Spectre of the Gun, The Enterprise Incident, Is There No Truth in Beauty?, The Tholian Web, That Which Survives, and Whom Gods Destroy) that I think are...okay. The other 16 episodes are garbage.