Choosing a lake in the north of Italy

Any of them are delightful.

The choice should be dictated by your overall itinerary.
If you are visiting the north east (Venice, Veneto region, Verona, Dolomites) it makes sense to choose Lake Garda, because it's in that area.
If you visit Milan, then you are closer to Lake Como or also Lake Maggiore/Lake Orta (Maggiore and Orta are next to each other).
Lake Lugano is also another, although mostly in Switzerland.
If you intend to visit the north west (Piedmont) then Lake Maggiore/Lake Orta is the obvious choice.

I expect that you will hear next other commenters suggesting one or the others. I have visited all of them multiple times and still I couldn't tell you which one I prefer. They are all gorgeous.

A few years ago, we had to choose between Como and Maggiore. In the end, we opted for the latter because it seemed to have more to do. There are the Borromean islands with beautiful villas and garden the lovely town of Stresa, and many charming villages. You can daytrip to Locarno. Mountain ascents via cable cars and similar are available in several places. I was trying to keep Lago d'Orta a secret; but since it no longer seems to be a "hidden" gem, I also recommend a visit to Orta San Giulio if you choose Maggiore. One more reason for our choice was that it seemed like Como accommodations were significantly more expensive, and that accessibility was more difficult, if you had a car.

I love gardens, so Stresa on lake Maggiore was the ideal choice for me when I was in Italy in May 2012. Think about the things that interest you, then follow your bliss.

No place is hidden to tourists in Italy. Least of all the lakes, including Orta.
A place may be 'hidden' to North Americans, who have not yet discovered it or even heard of it, but if it exists, the Germans have discovered it long time ago.