I arrive Paris on a Friday morning and will be staying in the 13th  arrondisement for 3 days. what you be the most cost effective transport ticket for the 3 days if I'm only be visiting zone 1-2 for the  duration of the 3 days . Thank you.

If you're planning on taking Metro/RER/Bus 4+ times per day, then your two best options are:

1. Navigo Day Pass 1-2 zone put onto a Navigo Easy card
2. three Ticket Mobilis 1-2 zone day tickets

Option 1 is better if you prefer holding / using a single plastic card to tap for validations.  You can only put a maximum of 2 Navigo Day passes onto the Navigo Easy at one time, so you will have to add the 3rd day's pass on day 2 or 3 of your stay
 
Option 2 is better if you don't mind keeping / using three separate paper tickets and discarding them as they expire after midnight.  You can buy all three at the same time (they don't start until you validate them in a ticket reading machine).  And you avoid the 2€ cost of the Navigo Easy smart card.