I don't thing there is going to be reliable information - the Christians in China keep their heads down for good reason as they've been slaughtered in the past.
That said, I've encountered so many devout Chinese Christians that I suspect that they numbers are higher - and given China's population, if only 15% were Christian, it would probably be the largest.
I think we also have to be careful with numbers too in the west - some people say there's 200 million Christians in the US. But I suspect a lot of them just tick off the box on the survey and haven't set foot in a church for years.
Here's a good explanation of how the numbers are always going to be suspect: https://www.quora.com/Why-do-estimates-of-the-number-of-Chin...
That said, I've encountered so many devout Chinese Christians that I suspect that they numbers are higher - and given China's population, if only 15% were Christian, it would probably be the largest.
I think we also have to be careful with numbers too in the west - some people say there's 200 million Christians in the US. But I suspect a lot of them just tick off the box on the survey and haven't set foot in a church for years.