I don't know what lake is visited the most by the population but I can say that Lake Eerie is on my own personal 'most visited lake' list. My family used to visit as a child and I have continued the tradition with my father on many occasions. The fishing is sublime. If you want to meet other people then I guess learning to fish would be a good start.

I guess it depends on the area and the attractions. If you are looking to fish, camp, hunt, ect.
I would say think of the accommodations you want most and go from there.

Lake Superior is noted as one of the best places to visit as far as fishing goes. If you are a fisher, or like the fishing scene, check that out. There are all kinds of surrounding attractions to see and take part of as well.

The wild blue of Lake Powell in southern Utah. The water takes the edge off the heat, and you float along, contemplating a lazy day of navigating spectacular red-rock gorges and flooded canyons. The gorgeous house boating was a fantastic experience you would enjoy. This is also the best place for honeymooners, family gathering events and social parties because they have wide and cozy hall and dining. At Lake Powell Resort, they offer a variety of picturesque boat tours, daily watercraft rentals, and guided hiking tours. Other area activities including dam tours at the Glen Canyon Dam, Antelope Canyon tours, guided fishing excursions, horseback riding, and more.