The first most vital area that is exposed as a target that I can competently hit given my skills, ammo, conditions, etc. I have shot deer in the head, neck, spine, and "boiler room". By far the most favorable shot for me is a broadside behind the shoulder blade about 1/3 of the way up from the brisket.
But sometimes a deer will stand behind cover and peer over at 35 yards, in which case I can easily hit the spine under his chin. Or a bounding away deer can turn and allow a rib shot that will penetrate the heart/lung area. Once all I could target on a nice buck was the base of the neck at the shoulder line at 70 yards. It all depends on the factors involved in any given situation.
Heart/lung shots are my most preferred simply because there is a greater margin of error: 2+ inches in any direction away from your aim point is still venison in the freezer. Not so with some of the other shots I have mentioned. The range and conditions narrow as the vital target area decreases with any given shot.