I think Africa is the continent where the church could get most converts atm.
However, Hans Mattsson (the Area Seventy who left the church) said that he was told that it's not worth it to send too many missionaries to certain parts of Africa since the people there who convert won't be paying much tithing. I can't remember where he said this, I think it was in an interview somewhere.
I've lived in several Sub-Saharan Africa countries spanning many years. I can say this: It's largely paper growth. In many Christian, Sub-Saharan countries people will "go for" many different denominations over the course of their adult life. Religious identify is very, very fluid. It's also very common for people to claim one protestant sect while still maintaining indigenous believes/practices.
Part of the problem is that the LDS church doesn't count members in the same way other Christian denominations do. This article sort of gets at that, but it's a reason why the General Conference numbers always represent a massive disconnect with reality on the ground. Despite the late start, the real story here is how the LDS Church isn't keeping pace with the Seventh Day Adventists, Jehovah Witness, Pentecostal Church of Christ, etc. per manpower, resources, etc.
I think the Mormon church will do well, by it's standards of measurement, in the English-speaking countries of Sub-Saharan Africa. But...the region will be an asset drain and their numbers will never match the reality.
I have travelled to quite a few African countries on behalf of the church and can attest to the ease of getting baptisms.
In fact, the church can have any number of baptisms they wish. They could easily baptise 1 million in one year if they wanted to show very significant growth. But, in my time they deliberately limited the growth for 2 reasons (1) the time it takes to train local leadership and (2) as Hans Mattson mentioned, the cost - African countries are and always will be a drain on church finances due to low tithing, high costs and welfare needs.
Moreover, given the JWs' long history of acceptance towards African-Americans (I read they make up one-third of membership) combined with the True Church's pre-1978 attitude towards those of African background, why even bother competing? Do they really think Africans aren't smart enough to find these things out?
It was awesome when I was in east Africa and my driver asked what religion I was. I said none but was a former mormon. He gave me a blank stare. I said white shirts and bicycles? Still blank stares. Joseph smith? Nothing. That is when I knew I was in a little bit of heaven, though it was fleeting.