I am arriving at CDG on Friday, likely going to Versailles and places inside Paris, and returning to CDG on Monday to fly out. Is it best to buy a pass or individual tickets, and what's the best pass to buy since it goes into a new week?
Arriving on Friday and traveling in and out of Paris (from CDG, to Versailles, etc.) your best option is a Paris Visite, 1-5 zone, for 3 days which will cover you for everywhere.
Note that Paris Visite will last only 3 days, so if you start to use it on Friday, it will cover you for Friday, Saturday, Sunday, but not Monday. You could also buy it Friday, but not use it until Saturday, and have it from Sat/Sun/Mon. Up to you how to make the most use of the card.
See Paris Visite article details for more information on how the Paris Visite works.