The answer is clear: Saudi Arabia.
Ever since the oil money started rolling in the Saudis have sent tens if not hundreds of billions on teachers, schools and mosques all over the world (Muslim and non-Muslim alike) insisting that the only Islam, the "true" Islam is that as preached and practiced by Mohammad in the 7th Century. The commands to kill non-believers in Mohammad as the Messenger of Allah are in Mohammad's Koran and in the reports documenting his actions in life.
Generations of Muslims have now grown up knowing no version of Islam except the Saudis' 7th Century violent version. Whatever toning down of Islam that occurred over the centuries (some, not much) has vanished in most of the Muslim world. Instead, every Muslim is taught that his mission in life is Jihad, helping Islam conquer the world and kill or subjugate all non-believers. Mohammad is proclaimed to be the "perfect man," the man to be emulated by all Muslims. This is a man who practiced mass murder, stole, raped and enslaved Jews and Christians and others who did not accept him as the Messenger of Allah.
Yet the Saudis show up at world terrorism conferences saying Islam has nothing to ISIS and has no relation to what the Saudis themselves preach and propagate worldwide. Nonsense, but they are not called on it.
The U.S. under Bush and Obama has shamefully let the Saudis get away with this hypocrisy and has not demanded that Saudi Arabia stop its worldwide incitement to violence in the name of Islam as the price of our being its protector (which the U.S. has been since the end of WWII.)
The Islam of Mohammad is the greatest threat to the U.S. and it cannot be effectively fought as long as our government continues to lie about it.
Read this report by Raymond Ibrahim of the recent security conference in Munich. And weep.