I would like to say kais deaths were sad but i saw them coming both times and the fact he was labelled iredeemable....it just came off more tragic than sad if you can understand that
Stefan, Hayley, Klaus, Elijah, Camille, Nora, Mary Louise ,Alaric, Jenna, Kai(saw them coming but as @Tommy,Klaus&Kol2012+ more tragic than sad) Bonnie's multiple deaths and Jo Laughlin's count for the most heart wrenching deaths , in my opinion.
Klaus and Elijah’s, also Damon’s at the end of season five when he’s saying goodbye to Elena literally makes me cry every time. Also Liz Forbes, I cry at so many moments when she dies and during her funeral.