Turnabout Intruder is abominable, and it's just a shame that such a stinker was the last episode of TOS to air. (In Spock's Brain it's at least possible to admire Nimoy's acting skills. His eyes look totally dead when he's without most of his brain.)
"Spock's Brain" has a lot more going for it than is generally appreciated. Sure, it's got a silly B-movie premise, but it's well-made. The actors treat the premise with great seriousness and elevate it. Nimoy's absence allows the rest of the ensemble to shine in a way they were rarely allowed to. The production values are excellent, with the rear-projected bridge viewscreen being a particular highlight. And Fred Steiner's musical score is perhaps his finest work for the entire series, as if he felt an obligation to compensate for the weakness of the material and sell it through the score. As ludicrous as "Spock's Brain" is, it's nonetheless fun and watchable. There are other episodes that are just dire and unpleasant, and "Turnabout Intruder" is definitely on that list. ("The Alternative Factor" and "And the Children Shall Lead" are there as well, and probably others.)