Love and hate are both emotions of passion. Indifference (or apathy) is the complete lack of passion, so for me personally, I find indifference has the potential to be the most hurtful. If someone hates you, you recognize that you are having a very real affect on their emotional state. The same can be said of love. Indifference, however, means that someone cares so little that there is no care at all, and therefore stands to reason that you have no affect on that person. If it's someone you care about or are trying to have a conversation with, indifference is by far the most hurtful approach.