What do they need to do? Why do they need to use Office? If you find something they need to do that is solved by an Office app, then show them how that app fills their need. If a user doesn't have a felt need for an app, then they probably won't take the time to learn it.

It does depend on the user, but I would normally suggest Word as the first application to look at. Of any of the applications it is the one with analogies to working without a computer such as letter writing, writing a book, creating a poster. All things that people might be used to doing, but where you can demonstrate some of the benefits of using a computer.