The hottest temperature ever recorded in the United States just happens to be the hottest temperature ever recorded!
This was recorded in Death Valley National Park on July 10, 1913 at 134 degrees fahrenheit. Death Valley is in the State of California. So based on the hottest temperature ever recorded California would be that state.
However, hottest average temperature goes to the State of Florida with an annual average temperature of 70.7 degrees (that's averaging day and night temperature!)
2nd place goes to Hawaii at an annual average temperature of 70 degrees,
For comparison the coldest annual average temperature goes to Alaska at 26.6 degrees (boy that's cold!)