There isn't any country specifically they are a powerhouse, let's say, where locals know their name, apart maybe on Japan, to some extent, since they just recently had a GP hit. But they are Top 3, and sometimes even 2, in popularity in most countries in the world within K-pop niche.
What I want to say is, their popularity isn't concentrate in a certain region like most groups. They have a relative amount of popularity in each part of the world. What makes hard to clarify from where their sales come the most. But I'd say: Japan, China, Europe, US and LATAM countries. For example, their sales had a big jump this comeback and I'd say the fact they started to have european distribution for the first time was a big factor (even though they debuted on the charts of all relevant european album charts only with digital + streams previously)
SEA and Korea are their weakest regions.
they're also quite well known among the middle east fans