The source is dated 2009, but it's likely to still be current. And it's hard to come up with a tourist destination that might beat the River Walk.
It wasn't always a nice river walk, though. It was once a dirty river prone to flooding. But in the 1930s an enterprising hotel owner convinced enough voters to raise property taxes to fund improvements along the river. Dams were built to control flooding, and eventually they got the river walk as it is today. Read more history here.
A lot of taxpayer money went into the project. One wonders whether it was a net benefit to taxpayers.
Via ijreview.com where you can find a U.S. map identifying each state's top destination.