You are allowed to go back in time and save a person from death. Who do you save? You can't save someone from old age, and you can't use the time travelling for anything else (just so we stay one topic). I'd personally save Cliff Burton, world's best bassist. He didn't deserve to get a bus on top of him.
EDIT: To everyone who won't save anyone because of how time will be messed up, this thread disregards any consequences. Also:
EDIT: To everyone who won't save anyone because of how time will be messed up, this thread disregards any consequences. Also: