I think THE WAY TO EDEN and SPOCK'S BRAIN are slightly redeemed by being so ridiculous as to be laughable.  One of my favorite bad TOS scenes is in SPOCK'S BRAIN when the effects of the "teacher" wear off in the middle of McCoy's surgery on Spock. 

"Captain... he's fer-getting!"

"All the ganglia... the neurons... I can't remember!"

For me the worst is probably AND THE CHILDREN SHALL LEAD because it's just depressing and weird and ends with a bunch of kids crying because their parents are dead.  Lots of fun there.