Tear Jerker: As usual, anything involving the Vulcan/Romulan sundering, particularly in Catalyst of Sorrows:
"Was the silence absolute, or did their ships sometimes...pass in the night? And if, when Romulus and Earth were at war and the Vulcans hung their heads and said "we don't know who these people are", was it at least partly true?"
The death of Hadlo, and also that of Bennek in Day of the Vipers. The former in particular: