What mobile wifi service is the best? At&t, Verizon, sprint, virgin mobile, other?

Virgin mobile indeed. I think it's $60 for unlimited internet
With the other companies you have to sign two year contracts and most of the those company's don't have unlimited internet which means you can go over and your bill can come out being hundreds more

All or none of them depending on your geographic location, all depends on their coverage maps in your area, I live way out west and far south along the border, and verizon and sprint have the best coverage way out down here.

If you had given your region (like which part of the vast US you are in it would have helped)