At this point it is never assured which app will be the best. This is because it is up to the company to continue to improve their product. Some companies believe that once they put out a product that everyone is buying, they will not need to do anything else.
They are the furthest from the truth. They always need to find better ways to improve their product. Therefore, WeChat could overtake Whatsapp someday, but it all depends upon the functions that will be needed to be added to WeChat to keep up with the apps that are out there. Also, WeChat needs to make it more useable to people all over the country. Therefore, WeChat make overtake WhatsApp if it can do the same things as WhatsApp.