I will be staying 10 days in Paris with a day visit into Zone 5. What is the best public transport ticket: Navigo Decouverte OR Origin/Destination OR T+ ?
If you only have one journey into zone 5, the cheaper alternative is to buy a Origine-Destination ticket for that journey.
To decide wether for the everyday travels you are better buying T+ tickets or a Paris Visite depends on how many different travels you expect to do daily and how they are done (see http://www.ratp.fr/en/ratp/r_61656/t-ticket/ for the rules that apply for connections with T+). Keep in mind that T+ is only valid for zone 1-2.
If you buy T+ tickets by 10, if you expect to use more than 25 tickets on 5 days, you're cheaper buying a zone 1-3 5 days Paris Visite.