I'll go for the realy dramatic one where Bobby asks Dean why he'd be so dumb to sell his soul to get Sam back (2x22). It was such a raw and heavy scene, and Dean on the verge of tears, voice quivering and Bobby wanting to tear his head off for what he'd done, but he can't because he can see Dean needs Sam so much.

Oh dear, I just can't! Didn't need my heart anyway...