Manual mode when you have the time, program mode when you don't. I leave the camera in program mode when I "put it away" so if I have to quickly grab it and shoot, I have a chance of getting the shot.
Something else I generally tell people is to take pictures in Program mode until it fails you. Why? It's a good way to learn the limitations of that mode and a good way to get a sense of aspects of photography that you may not be that well versed in.
Good example: A buddy of mine took a very close-up picture of me holding my camera in program mode. He got annoyed that the word "Nikon" on my camera was fuzzy, when my face a mere 4" behind it, was in focus. He asked what happened, I said he had a shallow depth of field because his aperature was wide open. He went "OH! That's what that means! Ok!" and then put it to manual and got the shot right.
There's nothing "wrong" with Program mode... it's just going to leave some of the thinking to the camera, and the camera IS going to occasionally make a mistake.