I can't stand how a number of people think "The Empath" was horrible. I thought it was terrific. The story idea was excellent. The aliens were intriguing. The imagery was convincing. It's definitely in my top 20.
Mine too, but for me George Duning's music is the main draw. Despite a script with its share of howlers ("Compassion for another is becoming part of her functioning life-system"), something about the episode elicited music from Duning that was poignant and at the same time metrically fluid and supple. The last act includes probably the longest musical passage without dialogue in all of Trek. I know some fans hate this music and have commented about it, but it's always welcome to me, whether intact in "The Empath" or cut up into pieces for episodes such as "Requiem for Methuselah."
George Duning wrote some of the best Trek TOS music, IMHO. His scores for "Metamorphosis" (one of my favorites) and the mediocre ep "Is There In Truth No Beauty?" are equally moving and memorable.
But then, in much of Season 3, the music outshines the scripts and the acting.