Think about the man hours that went into creating that dam. There aren't many things that we have created that even come close to equaling that achievement. The reason for its creation and the accomplishment itself speak wonders for what Americans can accomplish if and when they commit to a project.
I think that the Hoover Dam really is one of the most impressive things that we as Americans have come together and forged as a concept into reality. If you have never been there, then you really cannot marvel at just how amazing of a place that it really is.
While the Hoover Dam is indeed an engineering marvel and extremely impressive even today, calling it America's greatest achievement would be selling short the many achievements Americans have done. America has progressed so much in both sciences, as well as social policies that such progress itself is an achievement. I believe with how much the country has grown in both science and engineering, that is in itself a great achievement.
While I do indeed think that the Hoover Dam was a very impressive achievement, I think it was much more impressive when we as a nation put together a space program and landed men on the moon. The Hoover Dam was impressive for its time, but not any longer I say.