Yes the Stratosphere Tower is the highest and Tallest Freestanding observation Tower that stands in the United States.
The Stratosphere Tower has also been voted as the best Las Vegas Landmark and the greatest place to view the city from.
The Stratosphere is as of now considered Taller than the Space Needle standing at 1,149 feet tall and unless some other Tower is built in the future that is Taller the Stratosphere Tower is the Tallest.