Pullovers.
Machine
DB or BB
Variation is a good thing. Pullovers can be thrown in to back and chest days to really hit them. You really don't have to hit them heavy either. Like most other groups, good form is the key, drop the weight until you can keep clean form.
On a side note, you can use pullovers to help expand the rib cage as well. When doing pull overs on a bench with db or bb, once you get to what you think is your lowest point, pause, and fully exhale, this will help you to drop the weight a little further and will really help on expanding the rib cage.