Hard to rate this one, since it depends on what metric you're going for. The site itself is pretty enough, and Wizard's Mound offers great views of the surrounding countryside and St. Louis, but archaeological sites are always a tough sell unless you study the stuff. Also, walking around an open field at -15C was not super fun.

Besides the relatively boring tourist aspect (though the museum is nice enough), this is a really important site. Not just because it had been the largest city in the US until Philadelphia eclipsed it in the late 1700s, but because that city had been built by indigenous people. The size, importance, and longevity of the settlement kind of remind me of the early writings on Great Zimbabwe: white settlers couldn't believe that the "savages" they'd fought upon colonization could have been capable of building such magnificent earthworks. To those who with an open mind (beyond what is taught in school), it really blows open the concept of "American history" to a much longer and more interesting timeline. Basically, what is now the US was populated by sophisticated, cosmopolitan cultures that were utterly wiped out over centuries of violence fueled by a superiority complex. It's a powerful lesson, and the site is possibly the best example of this proud indigenous history in the US.