If you have read the EU, Luke essentially becomes God. He ends up on the edge of light and dark side with incredible power, calling on abilities from both while never turning evil. And in the actual canon, while he isn't as physically powerful or force sensative, he is still nearly as powerful, he is stronger willed and more disciplined. Leia is nearly as powerful, but never undergoes anything resembling training to hone her abilities. So in my opinion, Luke is the most powerful. But I am somewhat biased, as Luke Skywalker is my favorite Star Wars character.