It's difficult to determine the most popular restaurant in the United States as it can vary depending on location, personal preference, and current trends. However, some restaurants that are considered to be popular and well-known in the US include:

McDonald's: Known for its fast-food hamburgers, fries, and chicken sandwiches.

Starbucks: Known for its coffee, tea, and pastries.

Subway: Known for its sandwiches and salads.

Domino's Pizza: Known for its pizza delivery.

Chick-fil-A: Known for its chicken sandwiches and waffle fries.

Pizza Hut: Known for its pizza and other Italian-American cuisine.

Dunkin' Donuts: Known for its doughnuts, coffee, and other baked goods.

KFC: Known for its fried chicken.

Panera Bread: Known for its sandwiches, soups, and salads.

Outback Steakhouse: Known for its steaks, seafood and other American cuisine.

It's worth noting that this is not an extensive list and there are many other popular restaurants in the US depending on location, type of cuisine, etc.