Wondrous isn't it, that after all this time, after all of those viewings, that episode still has the power to reduce people to a torrent of tears. Young or old, man or woman, conservative or liberal, religious or secular. People from each of those fall under its spell. Not everybody, but a enough. Few other shows can claim that. Buffy can with "The Body". DS9 can with "The Visitor". TNG with "The Offspring". And there are a few more.
"Sleeping In Light" is the one that gets me most effectively, to be certain, but then it's the show that I have enjoyed the most. The characters I know the best.