The major legacy of Mustafa Kemal (better known as Ataturk) is the modern country of Turkey. Turkey is the most democratic and open society of any country in the Middle East with the possible exception of Israel. This is largely due to the way in which Kemal influenced the creation of the country.
After WWI, the Ottoman Empire was falling apart. It had not modernized in the way that European countries had. Kemal felt the need to change the society so that it would be more like the European countries. To this end, he introduced such things as democratic government, universal education, and individual rights. By doing so, Ataturk was able to create a relatively modern and open country. This is his major legacy.