Milk what do you interpret Will's line "I've never known myself as well as I know myself when I'm with him"?

Contrast. He knows what he is by witnessing what he is not. At that point Will was still fighting his nature, remember that he did try to kill Hannibal/rejected him again. It’s comfortable for Will to be with Hannibal because he can always compare his own darkness to Hannibal’s and at that point, Hannibal’s seemed darker. Will does that all the time, actually: define himself by association (whether better or worse than what he thinks he is).