What character death have hit you the most throughout the shingeki no kyojin franchise? And I'm talking about that one time big hit, like the character dies all of a sudden, but you weren't expecting that in that way (that's why I don't include Erwin).
For me it was Ian Dietrich, because he died while saving other soldier from a titan mouth while defending Trost (almost at the end), and with that move he sacrificed himself as the titan ate him, while Mikasa and Armin were watching from far and running towards Eren. What's most sad about it; he was highly skilled and ranked soldier (even killing titans in garrison), always remained calm and wanted to protect the main trio, because he believed in them. Soldier serving the right way protecting everyone. If he wouldn't sacrifice himself that day, I think that he would've had big impact at some parts of a future story. One of the most forgotten characters to me honestly.....