I'm gonna do something really controvesial and ill probably get shot for it.
I'm taking out City on the Edge of Forever.
Now WAIT WAIT, hold your fire! I do NOT dislike the episode. It's a perfectly good episode and a perfectly good plot and perfectly good writing. I just, personally, don't find it especially watchable or entertaining. It's not on of those I rewatch over and over and over. It's one I think 'hmm others seem to think that's a classic trek episode. I better put it on while I do this other more important thing so I can say I've watched it recently'.
You're not alone with those feelings. I see why people like it, but I'm not usually drawn to re-watch it.