Here is a secret:
The perfect golf swing does not exist.
Neither does the perfect jump shot, the perfect baseball pitch, or the perfect serve in tennis. This is because everyone has their own, personalized method of executing each of these actions; none of which can be deemed 'perfect'. If you gathered ten PGA Professionals and watched each of them hit a tee shot, then you were asked to pick the 'perfect' swing, you would struggle, because each pro's swing would result in (mostly) the same impressive outcome, albeit with different swing actions. A better phrase would be: 'how to achieve YOUR perfect golf swing', because a person's golf swing is a unique creature that is all their own.
The different ways to swing a golf club are truly infinite. There are 'flat' or 'one plane swingers' (think Cory Pavin or Zach Johnson), there are 'two plane swingers' (Tiger Woods, Fred Couples), there are 'outside in' or 'figure eight' swingers (Jim Furyk, Lee Trevino), and so on. How the club is brought back away from the ball is somewhat irrelevant, as is how the club is brought back down towards the ball. What is important is what path the clubhead is traveling on as it contacts the ball, the clubhead speed generated, and what direction the face of the club is pointing at contact. Much of this depends on what type of shot the golfer is attempting to execute.
Remember that a 'completely straight golf shot' does not exist. There will always be some movement either left to right, or right to left in the air, even if that movement is an inch or two (more often much more than that), it still exists. Therefore, golfers should not attempt to hit 'perfectly straight' shots. They should notice what their tendencies are (do you hit a draw or a fade more often on the range?) and plan, or 'set up' accordingly by either aiming slightly right or slightly left.
Generally speaking, a 'desireable' clubhead path is 'inside-out', or one in which the clubhead is approaching the ball from the left, contacting the ball, and releasing to the right. This puts draw on the ball (providing the face of the clubhead is either square to the target or slightly closed). The more common clubhead path for beginner and high handicap golfers is one that comes from 'outside-in', with an open clubface, resulting in a slice. This is where good instruction comes into play, as fixing a slice is an easier remedy than one might think.
So, the backswing and downswing are somewhat irrelevant (although there are some general checkpoints that are good to be aware of, and the weight shift/ posture/ ball position/ should be somewhat consistent) providing the clubhead is on a good path as it travels through the contact area and the face of the club is pointed in the right direction at contact.
