I chose The Land Before Time, however if The Secret of NIMH had been up there I would have chosen that. I have a 1982 VHS copy of it that I have taken great care of, more than anything I own. I don't know what about the movie makes me love it so much, but I do.
I admit, An American Tail was depressing. I honestly thought Fievel would never get back to his parents. There were so much false hope throughout the film, and I remember crying as a kid because I thought Fievel would never get back to his family. I can't say the same for The Land Before Time. The only really depressing moment, to me, in that film is Littlefoot's mother's death. The rest of the movie sort of picks up in tone after Littlefoot meets his friends.