The Dark Phoenix Saga- I think you just had to be there every month back then to get caught up in the soap opera and surprises. Art is great but the dialog is so wordy and it feels never-ending.
Kree/Skrull War- One of those early crossovers which was always cited, refenced, and had huge implications. But when I finally read it in my Marvel chronology project, it was such a let down.
Just giving my initial feelings, no offense meant!