In all honesty? The lower Niangua. But not necessarily because of the number of 18+" Smallies. It's because of everything else that most of us love river smallmouth fishing for.
Lack of party floaters, lack of jetboat traffic, awesome gravel bars spaced perfectly apart, big rocks, plenty of wood cover, clear cool water. The only deterrent is that access points are a long ways in-between (too long for most).....but that's its saving grace. π€
If you're strictly after Bigguns.....the Meramec and it's lower tributaries.