Suggest a good airport for France and Italy travel

It's likely that Paris and Rome will give you the most choices/best prices, but it really depends on your starting airport, and your planned itinerary in Europe.

I'd base it on where you want to be in Italy. Personally, my choice would be Rome over Milan as we like Rome better and you can fly direct home from Rome. If you have time, I'd do Venice to Florence to Rome and fly home from Rome.

Given the sizes of the 2 countries it would help to know what places in France and Italy you want to see, and what home airport you would start from. Flying into one city and home from another on a multicity ticket may be more practical and no more expensive than a return ticket into an airport that isn't one of your target cities.

Thank you all for your comments.

We are leaving from and returning to SFO.
We plan to visit Paris and Burgundy in France, so far.
Then Zurich to take the Bernina Express into Italy.
Go to Milan, Assisi, Rome, and Sicily in Italy.
We thought we would land at a central location, travel by train to Paris, then Burgundy, then Zurich, into Italy, perhaps a brief stay at the lakes, go to Milan for just maybe 2 days, then to Assisi, Rome and Sicily, all by train. Then back to the centrally located airport by train. About 5 weeks all together.
I welcome any ideas. Arriving and leaving from different airports is a possibility.
Thanks!

You can’t do from one airport. The best is a Multi city
SFO-CDG going
FCO-SFO returning.

Arrive Paris CDG
Train to Dijon
Train to Zurich
Train to St. Moritz
Bernina Express to Tirano
Train to Varenna (lake Como)
Train to Milan
Train to Assisi (via Florence)
Train to Rome
Fly to Palermo or Catania (Sicily)
Fly back to Rome
Fly back to US from Rome FCO

You could also fly from Sicily to the US, but not directly. You would need a layover somewhere (likely Rome FCO).
The best option for the above is AirFrance/ITA Airways. Both have non stop flights to SFO.