Remembrance of the Daleks
First time viewing here, and I enjoyed it a lot. Outside of The Five Doctors it's also my first time seeing Daleks in the original series, and I think they might even be more terrifying here then they are in the new series. I know this serial has ties to An Unearthly Child but I have yet to view that particular serial, or anything with the first Doctor for that matter.
They do gloss over this a tad, but the Doctor committing mass genocide is a pretty dark and scary place to take the character. Granted it's probably glossed over because it was the Daleks, and he did manipulate Davros into doing it...
I like Rememberance quite a bit and I think it holds up nicely with new Who, it feels more similar in execution than most classic episodes.