This is based on opinion. Please don't list it on a work's trope example list.
Tear Jerker / All You Need Is Kill
The fight to the death between Rita and Keiji. Made even worse by the fact that they are the only two people to truly understand the solitude of those caught within the time loop, so that whoever wins gets a double helping of grief.
Rachel's offer of herself to Keiji on his 80th loop, which he rejects. It's one of the beginning signs that Keiji's humanity is beginning to fade, and the way Rachel does it makes his rather brusque brushoff that much worse. The fact that she ultimately is one of the characters Killed Off for Real doesn't help things either.