I spared everyone on my last save, even Clarence (he dies either way), killed Playoby X, killed Francis and chose revenge. 
Ivan - Ivan was innocent and Vlad was a nobody, I didn't see any point in killing someone on his orders. 
Cherise - again, I didn't see any reason to kill her. 
Clarence - I randomly chose to spare him, but actually was going to kill him at first. 
Playboy X or Dwayne - Dwayne was a good guy and didn't deserve to die, also, the safehouse you get for killing Playboy is really nice. 
Francis or Derrick - I found this one easy, Francis was an annoying, whiny, crooked cop who kept blackmailing Niko into doing work for him, while Derrick was a good guy and a friend (he helps you in Three Leaf Clover). 
Darko - Why go through all this effort of finding Darko to kill him him and not feel any better, sparing him is the best choice to make. 
Revenge or Deal - I didn't want Roman to die, and Kate is lost either way.