I just finished reading Batman: The Long Halloween and it completely blew me away. At first glance I thought I'd hate the slightly caricatured art style, but now I find myself sad because all Batman comics aren't drawn like that.
The plot was very film noirish (loved the Godfather references in the first chaper) and intriguing. I don't want to spoil it to anyone, but someone starts killing members of the Falcone crime family once a month, every time on some special holiday. The mystery is built very well, I took several guesses at the murderer's identity, only to realise that I was wrong, then to realise that I was right and then wrong after all. The regular Rogue's Gallery was used well and I especially liked the appearances by The Joker and The Riddler.
To make a long story short, I recommend this to everyone, even those who don't normally like superhero comics. This one is more about the murder mystery and character development than action.