Williamson, TN in every sense of the world
Because the poster usually known as Extreme Conservative lives there? 
Del Tachi is alluding to an ideological sort of conservatism, rather than merely Republican voting habits, I believe. A ton of Americans are partisan, but only a much smaller number of people have coherent ideologies that go beyond mere partisanship. There are obviously a lot of counties much more Republican than Williamson County, but the sort of conservatism in Williamson County is a deeply ideological one (in comparison to somewhere like Grundy County, TN (a Kerry '04 - 82% Trump '20 county)), where Trump is almost treated like a religion, but fewer people could explain their political views coherently.
Williamson County is also kind of the poster child type of suburb for upper-middle class people who go to large evangelical churches. Just look at how much Williamson County stands out on this map of church attendance (as taken by GPS trackers on phones), even from the rest of the South. The Mormon Belt is the only place that seems to have it beaten. The type of conservatism that you get out of a well off Southern Baptist or non-denominational evangelical megachurch is very distinct from what you get in some Trump +70 rural county.
http://rynerohla.com/wp-content/uploads/2020/04/county_attend.pngAlthough, I don't know if I would describe Williamson County in 2020 as the absolute most conservative place in the USA, both because Williamson County has changed and (even more) because the definition of conservative has seemed to change. Ironically, some exurban county on the fringes of the Atlanta Metro might be the answer, even with how Metro Atlanta caused Georgia to flip. But, if we can rewind the clock a bit, the Daily Caller did rank Williamson County the #1 conservative-friendly county back in 2010:
https://dailycaller.com/2010/03/19/americas-most-conservative-friendly-county-is-williamson-county-horse-farms-guitar-shaped-pools-and-low-taxes/
