Best Place to Fly into/Stay for a Mont Saint-Michel Trip

and wondering the best airport to fly into

It depends on where your trip starts, but in general I have to say Paris.

I've looked into a trip to the Mont for several years and you really only have a few choices:
1) Fly into Paris and rent a car to travel to the Mont, this is probably the fastest.
2) Fly into Paris and take the train, this is not as efficient as I would like because of the connections.
3) Fly into Paris and take the Train to Bayeux as your home base, see the Normandy Landing sights, then take a day tour out of Bayeux that goes to the Mont. for a whole day trip, returning that night.
4) Fly to Paris and take a whole day trip to the Mont. returning to Paris that night.

In any case you have to start in Paris regardless of how you plan on getting there.

Paris is the biggest international airport, but then you have to travel to MSM. That can take the better part of a day.

There is an airport in Rennes, about an hour drive away.

Belgium is no where near MSM.

Belgium has to be a typo unless you have done no looking at maps. Do you by chance mean Brittany?

Donald's Option #3 would be my choice. I've been to those places and guided tours are both efficient and informative. A guide is essential for appreciating the WW2 landings.

Just returned from a visit to MSM where we took the TGV (high speed train) from Paris (best bet for an international airport) to Rennes (90-120min), then rented a car for a relatively short/enjoyable drive (about 1 hr) through the countryside to MSM. This was the fastest option we found.

We didn't have time in our itinerary for Normandy, however stringing together a couple stops in this region would have been my preference if we had a few more days. I would personally train out of Paris to my first stop, then pick up a car for the added flexibility.

If you are not big on crowds, I would strongly advise against a day trip, opting for an overnight stay on MSM. It's a much different experience at night and early morning.

A few other insights:

All TGV trains require seat reservations and do sell out. Book in advance.
Be prepared for the possibility of a manual transmission rental car.
If you explore the tidal flat, really pay attention to the tide tables. The warnings are not hyperbole.

Everything PNW Patrick said.