Voting for Death Note as a great combination of detective novel and horror thriller. The interplay between Light and L, and then Light and Near is fascinating, especially because later on they have direct interaction - and then there's Light's entire gambit to kill L by de-powering and then re-powering himself (Spoilers I guess).
The series really needs to have a more directed plot. Bleach has so many things to do that it would make a terrible movie. You couldn't even do, say, the Arrancar Arc because you have literally 10 people that have to be killed off in a fight scene, even if you cut out the psuedos in Hueco Mundo.
Contrast Death Note, which has a definite plot device, specific and important high points, deliberate shifts and ploys in the plot, and an ending which exploits the weaknesses inherent to the plot driver (the Death Note).
Another good one (I am serious): Bobobo-Bo-Bo-Bobo. This one would actually end up creepier than Death Note for all the zany crap that goes on, but its such an in-joke laden fun-fest with a ridiculous twist ending that, pulled off properly, it would be epic.
Finally, Big O would also work marvelously because so much of the important material is already consensed in a short amount of time, and the entire "Stage within a Stage" concept would fit the film genre very well.