I am going to go with ISIS. They are much more radical than the Al-Qaeda but just with fewer resources and less ability to directly attack the United State. While 9/11 was Al-Qaeda they were merely the ones with the ability to do it. If ISIS had the chance they would start launching nukes just to kill of the U.S.
Al qaeda kicked isis out of its establishment because they were too violent. Case closed. But i need to write a bit more to post this. I think extremists in general are quite stupid. The violence comes from poor education and social mobility. Thats why europeans join isis and are the mosts brutal fighters because they are alienated from cultural capital
So we've already got our answer.
ISIS now has much more operational capacity across the board to do damage internationally, and to hold their own in an actual confrontation.
Al Qaeda has already distanced themselves from ISIS for being too extreme and unruly, even for their taste.
Minor differences in ideology equal major differences in action, and we see with every knife soaked in American blood and every Yazidi family buried alive.