Usually when an actor has to be sad on the screen, they look more like they've sucked on a sour lemon. I'm not talking about mad crying. Usually a bunch of frustrated arm gestures helps make it a little more convincing. I'm talking about the sad crying, where someone is supposed to hold real still and let a tear run down their cheek and get a little lip quiver.
Since I watch entirely too many movies, I've seen a few actors who, to me, are good cryers. They do a great job at making me want to reach through the TV and ask them, "Do you want a tissue?"
Here's my list so far (in no particular order):
Jim Caviezel - High Crimes
Jensen Ackles - Supernatural (TV) (new addition that honestly surprised me)
Does anyone want to add one?