While both Errand of Mercy & The Quality of Life are fine episodes, I just like Errand a little bit more, guess you could say I have a soft spot for it.
1) It is the episode that introduced the Klingons, in general & the actor John Colicos to me. When I find a TV show or movie w/ Mr. Colicos, I want to sit down & watch his preformance.
2) It is a serious episode, with moments of humor in it.
“Errand of Mercy” (TOS 1x27)
“The Quality of Life” (TNG 6x9)