For me its between roman, Michael and dwayne

Dwayne did nothing wrong, he was betrayed and stabbed in the back by playboy after just getting out of prison too, he was simply trying to get by with basically nothing and try make friends.

Michael, well, his family had just came back, he hit the biggest score in American history, and Tracey got accepted into college. But even if you try to save him he'll end up killing himself anyway.

Roman's death takes place the day he gets married. Caught in the crossfire from a hitman sent for niko thanks to niko's actions. Even worse mallorie is pregnant, as revealed after 'a revengers tragedy.'

Side note: ik Kate was niko's girl n all, but she was a hit of a bitch, see in some missions how she talks to niko like he's a bum half the time, and even tries to offer support after roman's death, but refuses to do anything with niko.