Right now it is hands-down the United Kingdom. The UK is the biggest regulated jurisdiction in the world and has hundreds of licensees that serve various markets all over the world. Many online casinos will hold multiple licenses, but the biggest will generally hold that magical United Kingdom Gambling Commission seal of approval. Having said this the United States and to a lesser extent India (because the economy is not worth as much per head) are the sleeping giants of the online gambling industry. The USA has only just began regulating online casinos in most states, while India and its population of close to 1.5 billion is yet to put anything in stone, but have began discussions about regulating some forms of gambling.