"The Trouble with Tribbles"---I don't hate it, there's actually no episodes of the original series I hate. But it's noted by a lot of fans and casual fans as a favorite. It's ok, but I usually only watch it when I'm doing a series watch through. It's not something I'll pull out at random to watch like I would something like "The Doomsday Machine" (I love the remastered version of The Doomsday Machine BTW--hell, even the original was kick ass).
A lot have mentioned "Spock's Brain" as a guilty pleasure. I'm in that camp too. I even get into "And the Children Shall Lead" and for all it's cringe worthiness I count "Plato's Stepchildren" as a guilty pleasure. "Turnabout Intruder" too, yeah. For all it's sex discrimination aspects it was sort of a classic old fashioned sci-fi story. As an aside I remember a novel tried to address that, that Lester was passed over for command not because she was a woman (after all we know now women were indeed in positions of authority thanks to Enterprise and Discovery), but because she couldn't pass the tests. She convinced herself it was due to her being a woman and Kirk finally gave up trying to convince herself otherwise.
"Catspaw" was a personal favorite. I mentioned this once before but when I was a nubie fan back in 1986 my local supermarket had several episodes of the original series for rent and that was really my first exposure to the TV series after I saw the movie. They had "Catspaw" as a selection but for some reason it wasn't in stock for weeks and weeks, I guess someone forgot they had it. One day I saw the box out for rent and I was thrilled, but sad to find out someone accidently put the box back out, it still wasn't returned.....then one day it was back. I was so thrilled because I had already rented the other 10 or so episodes they had at least twice and "Catspaw" was the only one of the bunch I hadn't seen yet. Finally I got to see it----so I always have a certain fondness for that one.