ER: When Dr. Greene dies. Much of the filming was done here, and dammit, whenever they play IZ's "Over the Rainbow/What a Wonderful World" just turns the waterworks on.
Gilmore Girls: When Rory graduates from Chilton and gives her Valedictory speech
Grey's: When Denny dies and Izzy's mourning thereafter
Sesame Street: I recently saw a clip somewhere of the episode where Big Bird learns of Mr. Looper's (Hooper) death and I teared up. I was still a very young kid when this happened, but I remember it was the first time I learned about death. I think I was about 3 or 4 years old.
Pretty much every Grey's episode - well, not the new ones, but the Denny dying/George's dad dying episodes.
Now I also cry when I see the occasional TLC's A Baby Story - probably because my pregnant lady hormones are kicking, but also because I think that whole birthing thing is starting to worry me!