I think for me, the saddest episode is when Ben dies. I really felt for Dr. Cox during that episode and the few to come, which is not much I can say for other TV shows.

I think it has a nice blend and mixture of every feeling throughout the seasons. It can be funny, make you happy, to making you sad and feel for the characters.

Anyway, that's just me. I find it hard to watch anything on TV these days, with this being the exception.