Well, it depends on your product and finding your niche. Even in a single country, there are different regions with a different specific product that is marketable to them. You have to know your market first before doing business. It's not easy and takes a lot of time, but once you identify your winning product for a specific location, it will be all worth it.
Although there are countries with restricted access to the internet,I don't think there is the best country for online marketing. It just have to be the best marketer. It may seem that America will claim one of the top spots but you have to know also that not everything that is advertised in America is successful. Same with other countries. So there is not best country for online marketing in my opinion.