I can work down the coast to some southern Atlantic states. Counties are colored based on my personal visits (green=yes, red=not yet).
SC: Allendale (10419) and Barnwell (22621). Allendale was the 2nd least populated county in 2010 and is now estimated to be the least populated. The Main Hwy (US-301) misses Barnwell, and much of Barnwell is the Savannah River Site which most people can't visit anyway.
Given the population and the history, I'd actually merge both Allendale and Bamberg back into Barnwell County. At only 45,000 people it'd still be only 26th out of 44 in population tho it would just barely be our largest county, slightly bigger than Charleston which is spread out along the coast.
Merging McCormick into Edgefield, Calhoun into Orangeburg, Hampton into Jasper, and Lee into Sumter also strike be me as good choices and that would get rid of all the counties that have less than half the population they'd need to be made as a brand new county today. (As a relic of when our House of Representatives was apportioned by counties and each county had one Senator, the requirement is that a new county must have 1/124 of the population of the State since we have 124 Representatives in the General Assembly.
All these would also have the advantage of eliminating all of our counties named after Confederates.