Dial M for Murder
Great first half, second not as good.
It is an interesting mystery where you as the audience actually know everything beforehand because you follow the wannabee murderer for the first part. Therefore you get in on his plan and that is very interesting. The second part does not work as well because you really know what way it is going to go and it takes just a bit too long to get there. The first is really interesting and thrilling but the second just seems like they need to get the story all wrapped up.