Mordin Sacrifices Himself
Mass Effect 3, Priority Tuchanka
Not going to lie, I was deeply affected when Mordin started singing his "Scientist Salarian" song. It's even worse if you play renegade and can't convince him to let the STG sabotage run its course (if you killed Wrex and/or Bakara dies). Nope, I'm not going to link to that video because it hurts my heart just a little bit more than I'd like to admit.
Later on, in the Citadel DLC, Shepard discovers a tablet with several audio recordings, which seem to actually make his death that much more poignant. He was such an incredibly complex character - arguably one of the best written from the whole series - that, the more you found out about him, the more you wished he would have survived. But like he said, it had to be him.
"Someone else might have gotten it wrong."
Mordin's Final Goodbye