In terms of potential i agree on India and China too.
In terms of actual representation it's obviously brazil or indonesia.
Geographically asia is the place to be when it comes to bitcoin.
Idk about that though... There are a lot of barriers to cross when it comes to major adoption in India and China.
For one with India, there is only a couple of cities there that have people with nice phones, computers, and are technologically capable of doing business with bitcoin. Does this mean that they will do business with bitcoin right away? Of course not. There has to be an incentive for wanting to do business with bitcoin, and bitcoin is way too volatile for people who are living off of a couple dollars in rupees a day. If bitcoin falls out of the sky, then that would probably mean that they would lose their business and they are shit out of luck. As of right now it's all a gamble for doing business in Bitcoins only. Also, if I'm not mistaken, there aren't too many exchange sites for people in India to exchange for rupees. Not to mention that we are just talking about a couple of cities... a lot of India are very poor areas who are doing great just to have electricity.
As for China, it comes down to regulation of the government. If they continue to allow bitcoin to be bought and used by the Chinese, then yeah it will be great for bitcoin... but from what I've been reading, China is becoming a lot stricter and are going to be enforcing rules in a really different way. Look up "Seasame Credit in China" for more info... They are allowing it now for any one who wants to participate and are going to enforce that every citizen partakes in 2020. Basically, it makes everyone become a slave to the government, and if you as much as speak out against the government, then all of your rights are taken away essentially and those that do speak good about the government will be "rewarded" with benefits like being allowed to travel out of the country more frequently and other stuff.
There is a lot to consider which will be the next major nation that really accepts Bitcoin in general, but I'm surprised no one here is talking about the Isle of Man?