What is the best metro/bus pass to visit Paris for 2 days while staying in Rueil Malmaison.

While the Paris Visite is the most expensive product for transport, since you are outside the metro system and will need to use the RER to get into Paris, you will either need day passes which you can load on a Navigo Easy card or a Paris Visite.

The day passes for zones one and two are about 9 Euro a day. A 3 day visite for zones 1-3 is about 30 Euro. With a pass you will have unlimited rides in that zone that day -- they are done at midnight so you need to be back by then if you have a day pass. The 3 day pass of course is not done till the third day midnight. Since you will be there 4 days with 3 nights, you wold probably have to get a 5 day visite -- they don't sell 4 day passes.

On the RER you have to use the pass to enter and exit. on the metro you only need it to enter and if you are stopped by a ticket checker. This happens fairly rarely, but once I had it happen 3 times in one day. We average once or twice a week whle in Paris.

Rieil Malmaison is a a rail desert. Looks like you will need to take a bus just to get to the RER unless you are at a hotel by the Rueil Malmaison RER station.

You are in Zone 3, so the Visite 5-day is 44.45 EUR. You can buy Navigo Day passes zone 1-3 for 11.60 EUR.
That brings up the question, how are you arriving at Rueil Malmaison and how are you leaving? From/ to airport, train station, taxi?

Your best deal would be a Weekly Navigo Pass for 30.75 EUR plus the 5 EUR initial set-up fee. You need to bring a passport type photo, 1" X 1 1/4".