I am the worst sinner I know, simply because I know myself better than anyone. My sin is the worst because it is mine. I am intimately involved with it. I know all its subtle nuances, its illusions, its rationalizations, and its cover-ups. Of my sin I am an expert.
The remarkable thing is not that I sin, but that in spite of my sin, I am capable of having fellowship with God and being used for his purposes in the world.