Most extremely conservative county in the US

[I’m not Stu]

What county is the most extremely conservative?

[I’m not Stu]

La Salle Parish, LA for racial conservatism/general reactionary-ness, from what I’ve heard.

Yes. It voted for a white Democrat over Bobby Jindal in 2003. It's David DuKKKe's best parish.

[I’m not Stu]

Williamson, TN in every sense of the world

Because the poster usually known as Extreme Conservative lives there?

ExtremeConserative (and Blairite) gave us really good posts upthread, and what I was thinking when I named Williamson was a more "cultural" conservatism that goes beyond the Trumpian Republican politics of our moment.  Williamson has some of the biggest Evangelical churches in the country (as alluded to), the population resides almost entirely in SFHs headed by married couples, is ~90% White, high income, and exhibits high levels of trust in social institutions (churches, schools, community groups, etc.)  I also like Blairite's suggestion as Baldwin, AL as a potential answer, but the historic French Catholic culture in Lower Alabama/Mobile diminishes Evangelical influence in the area a bit IMO.

Rural counties mentioned by other posters may be more Republican or "Trumpian" but they are not more "conservative."  They are poor, uneducated, have remarkedly low levels of social trust (church attendance throughout rural Appalachia is abysmal) and have a much higher proportion of single-parent or mixed households than places like Williamson or Baldwin.  

This is why I would also stick by my pick of LaSalle Parish, LA. It is not only demonstrably one of the most racially conservative places in the country, but it also has deep (for the non-ancestrally Unionist South) Republican roots, and started voting Republican at a time (the 1950s) when in the South this in large part indicated strong economic conservatism. But I agree Williamson (and in that vein Montgomery, TX), or at least its incarnation 10 years ago, is a decent bet as well.

Bobby Jindal, the economic conservative candidate, lost LaSalle Parish in 2003 and severely underperformed in the parish in 2007 (Democrats got over 40% in 2007). Racial conservatism is a higher priority than economic conservatism in LaSalle Parish.