|Mae Hong Son||275884||43918||15.9%|
The lowest percentage and absolute number is found in Yasothon - as can be seen in the map, most of the northeast and north have less than average foreigners. Even though this population data isn't published in the Royal Gazette yet, I have used the numbers as of December 31 2016, which can be found at the statistics page of DOPA.
To confirm the assumption that its mostly people from Myanmar, one has to look into the census 2010, which includes the list of nationalities found in each province. For the whole country, 1292862 people from Myanmar were counted, 281321 from Cambodia, 141649 from China. The only other groups above 100000 are those of "other country" (120699) and those with unknown nationality (117329).