This is based on opinion. Please don't list it on a work's trope example list.
Tear Jerker / Homicide: Life on the Street
Giardello's speech at the end of "Partners and Other Strangers".
The scene in "Every Mother's Son" when the mothers of a murdered child and the boy who did the killing meet unaware of who the other is.
The scene in "Crosetti" when Lewis realizes that his partner and friend committed suicide, and also Pembleton's one-man honor guard at the end.
"A Doll's Eyes" when the parents have to take their son off life support.
Charles S. Dutton's performance in "Prison Riot" especially his line "I'm in here forever, Detective. Forever."
The end of "Blood Wedding" and "Betrayal".
Bayliss' shooting in the season 6 finale, "Fallen Heroes". Just all of it from Pembleton's reaction to the look in Bayliss' eyes (and his eventual seizure) to the former's breakdown in the hospital, begging God to let his best friend live.
The end of "Betrayal" in which Bayliss reveals why he takes cases with murdered children so hard.
Lewis talking Kellerman out of suicide in "Have A Conscience" with a Heartbreaking invoking of Crosetti.
"Bop Gun" is one long Tear Jerker with Robin Williams as a man who just saw his wife murdered. Highlights include him becoming furious after hearing the detectives humourously banter about the murder and the investigation and the final moments in which he says he is now a member of a "special club where only the initiated can recognize the other members".