Personally I would go with The Great Gatsby, as it is one of my favourite books of all time. It was also the book that I was made to study at school that I actually enjoyed!
One of the reasons I liked it so much is because I am interested in 1920's America. Prohibition, the "flapper" culture and the extravagant parties are evoked vividly in The Great Gatsby.
Nick is an intriguing narrator, and seems as in awe of the lifestyle as we are. Whilst Gatbsy, the eccentric protagonist is a divisive character who is loved and loathed by the reader, often at the same time.
I believe The Great Gatsby is the quintessential novel about the destruction of the American dream. It is laden with symbolism, and is the kind of book that warrants a second or third read to pick up on things you perhaps didn't notice first time around.
The Baz Luhrmann film about the book is, in my opinion, less impressive, although it is worth seeing purely for the visuals of Gatsby's famous parties, depicting the glamour and the decadence in typical Luhrmann style.