These are the four TOS Season 3 episodes that contained some good things, but are weighed down by some parts that were truly awful.
Way to Eden: 8 minutes of horrible "space rock" plus another 8 minutes of a boring Checkov romance. The other 34 minutes aren't bad. The badness starts at the 11:00 minute mark, and goes on for another 5 minutes. Then goes off the rails again at the 24:00 minute mark till the 35:00 minute mark,
Spock's Brain: We get an interesting first 21 minutes. But the second the elevator reaches the ground floor its snoozeville for the next 15 minutes. This is where we get lots of boring fights, and lines like "Brain? What is Brain?". Its not till Zombie Spock releases the "Pain belts" that the episode gets back on track. The ending surgery wth McCoy is quite well done.
That which Survives - In terms of rewatchability the first 15 minutes are at the bottom of the TOS episode rankings. Its not till D'Amoto dies, that it starts to get interesting. Before that its just exposition and padding. By starting to watch after his death, you miss nothing. The rest of the episode is well done.
Plato's Stepchildren: This episode has more bad mixed in with the good than any other TOS episode. We get 14 minutes of completely unwatchable "torture porn". Bascially, two 7 minute long segments, one in the second act, and one in the last act. If this isn't bad enough, the first 6 minutes are boring exposition. Its not till Parmen becomes delerious and things start flying around that it becomes interesting. Now, on first watch its not uninteresting, but once you watch it once, the only thing you can do is is FF through these 20 minutes, and watch the remaining 30 minutes.