wow the mainstream anime is real here, well mine is probably a tie between ouran high-school host club and maybe soul eater although my favorite echi has got to be kissXSis
One Piece is an adventure story with a lot of action, comedy, superb character development and good story telling. It is very long, but since it keeps superb quality throughout that's a good thing.
Mushishi is a thoughtful, episodic fantasy anime with a focus on folk lore, fascinating fantasy creatures, beautiful music, and questions about what it is to be human.
In terms of sadness, the Clannad without a question. In terms of emotional roller coaster, tie between Peach Girl and Highschoo DxD. (Yes i get very invested into the characters). In terms of general favourite Say "i love you" nice short series you can watch in a day