Spider-Man - Amazing Spider-Man #39-40, 90, 121-122, 229-230, 248
Spider-Man: Blue
I can't remember what comics these arcs were in, but Who Killed Jean DeWolff, and Kraven's Last Hunt are also defining moments and requird reads for the Spidey fan.
Batman - Year One, Death in the Family, The Killing Joke, The Man Who Laughs, The Long Halloween, Arkham Asylem: a serious house on series earth.
A new addition to the Bat list I think will be Monster Men.
I consider it to be, but haven't heard to many say it is yet.