Are F5 tornadoes the worst

Generally yes. F5 is the strongest category of tornado capable of completely obliterating well built structures. However how bad a tornado is depends on factors other than strength, such as where the tornado hits. For example, a number of F5 tornadoes have traveled over open country, with the worst damage limited to farms that were destroyed.

A list of the worst tornadoes in U.S. history will consist mostly of F5 or EF5 tornadoes (including the #1 worst), but several F4 tornadoes would be on that list as well.