What kind of ticket do you need to reach La Défense?

You can take Metro 1 from anywhere in Paris to La Défense Grande Arche using a single ticket.  Upon arrival, be sure to exit using Metro exit turnstiles as the Ticket t+ won't let you exit by RER exit gates.  This is only a possibility when at times the station authorities open barriers normally separating the two systems allowing freer flow of foot traffic in and out of the system, usually during rush hours.