We all have one. What characters death felt unnecessary, was just to painful to watch, or just gutted you to the core?
Now, I know people often say Mordin, and it's definitely a strong contender.
But IMO even worse is the death of Wrex if you actually betray him.
I did it once to see what happens and immediately reload after the scene with him on the citadel.
Mordin died for a good cause, Wrex died because you spit on his friendship, doomed his people and he dared to hold you accountable for it.
Wrex death is YOUR FAULT.
I'm also going to throw in legion on the paragon side. Mordin's death is, pretty inherently, redemption. Legion, on the other hand, finally achieves its/their goal. They're an individual, they have a soul. And then, it must give that up and sacrifice itself so all the other geth can have the same opportunity.
"Legion, the answer to your question was yes."
"I know, but thank you."