What is the magnitude of the largest earthquake in the US?

Magnitude 9.2 including Alaska and 9.0 in the lower 48 states.

According to USGS National Earthquake Information Center it was magnitude 9.2 on March 28, 1964 in Prince William Sound, Alaska.

On January 26, 1700.was the largest known earthquake (magnitude 9.0) to have occurred in the "lower 48" United States that rocked Cascadia, a region 600 miles long that includes northern California, Oregon, Washington, and southern British Columbia