According to mainstream economists, the major beneficiaries of NAFTA are consumers in all countries. Mainstream economics holds that free trade brings benefits to the average consumer everywhere.
The reason for this is that free trade allows all countries to benefit from what each country does best. If Mexico can provide cheap labor, that helps consumers in all North American countries because they can now get goods at a lower price. If farms in the United States can produce food more cheaply than farms in Mexico, consumers in Mexico benefit. They no longer have to pay as much for food and therefore have more money to spend on other things.
Economists would argue that free trade agreements such as NAFTA bring the most benefit to the average consumer of every country involved because that consumer gets to have access to more goods at lower prices.