The Immunity Syndrome:

Possibly the most boring TOS episode ever made.

In the words of a friend of mine, the TOS episode that should've been shortened
to a 30-minute Animated Series episode.  (We talk about TAS a lot.)
 
This episode is interesting in that it may possibly contain the inspiration
for Obi Wan Kenobi's "millions of voices and suddenly silenced" intense
clairvoyant pain he experiences.