This is based on opinion. Please don't list it on a work's trope example list.
Tear Jerker / Vampire Academy
In Frostbite, Mason's death especially how he ran to help Rose and was just killed with no mercy by the Strigoi. Then when Rose is just cradling him and has to be pulled away and when she begs to stay saying that she has to protect him and Dimitri tells her she already has.
Near the end, when Mason's ghost appears one last time to say goodbye. (And he smiles).
The incredibly sad ending to Shadow Kiss, when Rose finds out that Dimitri has been turned into a Strigoi.
Rose spending her time in the Academy's church after the rescue.
In Blood Promise, Rose staking Dimitri.
Spirit Bound is worse, with Dimitri's line "Love fades. Mine has."
What no feels for poor Adrian. The guy funds Rose's expedition to hunt down Dimitri, tries to quit smoking and drinking (despite him needing that stuff to combat spirit's affects), and partially alienates his family and she still cheats on him with a guy she said she was over. Damn!