Standoffish - I believe this is a stronger term used when one is considered antisocial Aloof is more related to shyness Indifferent is used to indicate that one does not care about something (a synonym of apathy)

Standoffish implies a little rudeness- that someone is rudely avoiding someone. Aloof might also imply rudeness. That the person thinks the are better than the other person so they don't want to deal with them. Indifferent is more nuetral. The person might just not care about the situation but not be making a judgment.