For me that honor goes to the Lost finale, lol. I did like Six Feet Under but only viewed the first two seasons.This is an amazing series. It started going away from what made it so fascinating, and I often wondered, "it's not broke, why are you trying to fix this?"
The ending to this show is killer. I have never cried so hard over a season finale in my life -- before or since. Of course, I had a lot of things going on in my life that contributed to my view of life in general, so that may have had something to do with my reaction. But by what I heard, most people had the same reaction.