You must watch Clannad and Clannad After Story. Of all the movies, TV shows, and anime I've seen, it is by far the most touching. You will cry, and you will cry lots, just don't fight it.
EDIT: And also, despite what everyone else is saying, Clannad is not depressing, the message of the show is an extremely happy and heart-warming, although it does have it's very, very sad parts.