Well they all left me sad and with that numb feeling that I don't seem to get with deaths on any other shows unbelievably. I mean I thought about Fred's death for weeks after the episode aired it disturbed and upset me that much, I was in shock at Tara's and oh my god Jenny's! That was like first major death (since Buffy was brought back to life in Prophecy Girl) so I was all 'No one's safe. Oh my god, he actually did kill her, they didn't save her!' and Giles' reaction had me crying my eyes out (Apparently not many of the crew could bear to watch that scene where he breaks down outside the factory- they all found it too uncomfortable!).
But perhaps not surprisingly the saddest death for me was Joyce's. On a basic level "hey they just offed my favorite character!" but it was the whole episode dedicated to it showing each reaction to death (and think that was another point as well. It wasn't someone dying we were dealing with like when characters have been killed/risked lives etc, but it was actual Death we were dealing with here) and many episodes after that (on into season 6 and 7) where they constantly jabbed the knife in to show how it had effected and left a hole in each of the character's lives. Plus the way it literally hit home was a little too much for me. I don't know what I'd do if my Mom suddenly died like that. So that death hit me on so many levels that it's impossible for me to not say it was the saddest.