Which lake

I wouldn’t choose based on the flatness of the land. All those lakes have towns that are flat and towns that are not, lake Maggiore is surrounded by mountains and hills too. Therefore more than the lake, it is the individual towns that have different terrains.

All those lakes are beautiful. You would probably prefer Maggiore, since you have not visited it yet, but your husband will enjoy any you choose. When making that choice you may want to consider the overall itinerary you intend to pursue. For example if your travels entail the use of Malpensa airport, then Lake Maggiore makes a lot of sense. If your travels include the Dolomites, then Lake Garda may be a sensible choice.

On Lake Maggiore the mountains are set back more than than on Lakes Como and Garda.
On Maggiore, I think Stresa has uphill areas. Ranco on the east side was flat , great hotels and restaurants My husband has a severely bad back and we have gone to all these lakes. We have returned to Lake Como four times even though it is very hilly.

Your two good choices are Maggiore and Orta. I like both. Depending on how long you are staying you could base yourself on Maggiore (Stresa is convenient and plenty of hotels and restaurants. Then perhaps a day trip to Orta.

I arrived in Stresa by train and walked from the train station to my hotel then to the center of town. Fairly flat

If you stay on Maggiore, make sure you visit Isola Bella--an island in Lake Maggiore. Fascinating palace and gardens.

I've been to Maggiore, Como, Orta and Garda....every one is a treasure, and you can't make a mistake with any. I took an easy, easy, easy express bus from Malpensa Airport to Stresa - I think it only took an hour. door-to-door. From Stresa we ferried to Isola Pescatore on the lake. Orta is a smaller lake; if you like Alessi products, there is a factory store near Omegna with good pricing; south of Orta San Juilia, I had the lightest, best lasagna of my life in a little waterside restaurant. We stayed in Como a couple of nights and ferried to Bellagio and Lenno for a few hours. And on a different trip I took a train from Venice for a daytrip to Sirmione on Garda. I don't think you can go wrong with any of them. Enjoy!

We landed at Milan Malpensa airport, picked up a rental car, and made the easy drive to Stresa, where we stayed for 5 nights. (Day trips from Stresa.) My back is fused from my neck to my tailbone and is full of rods & screws. I didn't find walking around Stresa to be a challenge. It also has any kind of restaurant/shop that you could possibly need.

That being said, we made a day trip to Cannobio and loved it.......Quieter, more charming and more Italians vacationing than Americans. We definitely want to return! However, for convenience, Stresa is much more accessible to the ferries, restaurants, etc. One of our day trips was to Lake Orta.........Lovely, smaller lake with a charming town at the ferry dock. There's a little island in the middle of the lake. The only structure on the island is the most amazing Benedictine Monastery. We attended a Gregorian Music concert in the church, which was the highlight of our stay!

Given that we had a car, we also made a day trip to Zermatt, Switzerland. Another amazing experience.........Driving over the Sempione Pass felt like we were in the Sound of Music. Of course, the Matterhorn is breathtaking! And since we arrived in Stresa on the first day of the worst heatwave to hit Northern Italy in decades, the cooler temperatures in Switzerland were a welcome relief. We're now determined to return to Switzerland for a 10 day road trip!

Hope you have an amazing trip! :)