Usually for me it is either to completely unplanned picture or the picture that I had to work to get the shot that I like the best. The image below is the exception to that rule.
I had been wanting to get this picture for a long time now. For one reason or another it never seemed to work out. Earlier this year part of Tioga road was closed down due to the rim fire. This prevented people from the central valley and coastal regions of California from having an easy way to get to the Tuolumne Meadows area of Yosemite. This, in turn, enabled me to get in a last minute trip to the area in September of this year.
The second night I was there I drove over Tioga Pass and down the road a little way. There I setup my camera, framed the picture, took a couple of shots to get the exposure I wanted, and then waited for a couple of cars to come up the road. As soon as I saw a car at the bottom of the road (just out of frame) I opened the shutter and waited. a few seconds into the shot car came from behind me driving down the hill (you can see the red tinge in the light streak in the shot.
Camera: Canon EOS REBEL T2i
Lens: Canon EF 24-105mm f/4 L IS USM
So while planned the shot was almost easy to get for me that night. Things just worked out. I ended up taking a couple more pictures just to be save but this first one was by far my favorite.