I think the basic idea is to show through your profile that you're a competent developer. The project you pick doesn't necessarily need to do anything new and amazing, it just needs to be your work. You could make a task-list app (which has been done a million times) but if you do it well then you've achieved your goal.
Of course, the best outcome would be to make something that other people want to use. This would get attention and further validation. So if you find a project you're passionate about---the "I built it because nobody had yet" thing---then that would be best.
Good luck!
