Right now Gatsby, but I don't really know about any other movies that are supposed to come out this year, so I guess it's more Gatsby by default than anything else.
As far as books go, Gatsby should be relatively easy to turn into a movie, I think. Just shoot the thing the way it's written, don't try to change the plot or anything, maybe even take some dialogue word for word from the book. The less fancy they try to get with it, the better