So imagine you are Shepard. You did all you could, saved as many people as it was possible and you've made your way to the end. But then the Catalyst tells you that it (somehow, don't ask) has the ability to bring back two people you knew, one you were close with and the other you didn't know all that well (aka, some side quest NPC). Which ones would you pick? Assuming that everyone else that can actually be saved in the game is alive as is, so pick those whose deaths are completely unavoidable (but we can count Mordin for this since can only live if Wrex and Eve both die, so for this scenario, it would be Mordin surviving alongside them; also you can save Ash or Kaidan as well, just imagine yourself which one you left on Virmire).

For a major character my pick would be Anderson. The dude makes it almost to the end, and while most other major characters that die usually do so via heroic sacrifice, his death is rather undignified as he just gets controlled and shot by you, also controlled. He also never got to meet up with Kahlee again. Narratively speaking I don't mind that it was written this way, but from the emotional standpoint he was always the one character who I wish got to live and see the war end.

For a minor character, I'd pick Charr (you know the blue rose of Illium guy who you find dead during the Aralakh company mission in ME3). I know it might sound weird and random but for some reason his death made me especially sad (even moreso if you help him and Ereba get together).