Some might think certain elements of "A Christmas Carol" would qualify as "horror" but  I would suggest perhaps the most powerful and disturbing "non-horror" episode would be #1076 "Star sapphire". (WARNING: there is implied child abuse, definitely emotional and borderline sexual, in this Elspeth Eric tale...and to some that would qualify as "horror" and I wouldn't blame them. This episode made me want to adopt a poor child and give him/her a home, as my own Mom was herself adopted.)

For the second most powerful "non-horror" RMT I would suggest episode #1096 "Life blood". This one will bore some as it's nearly all melodrama, but in the wonderful old (if you can call the mid-2000s "old", sure is in Web terms) CBS RMT website this was the most discussed the episode ever in terms of sheer number and length of responses. It's fascinating - the writer was the same guy who wrote the original screenplay for "Casablanca", but for inspiration he clearly took the original "Twilight Zone" episode "The lonely"...not surprising as that show (and its incredible music) inspired other RMTs...yet this show was "The lonely" told completely in REVERSE in all aspects, which made for an interesting hour. 