Obviously, the best ball is the length ball just outside off stump. It's the ball that most bowlers are trying to bowl most of the time, regardless of skill and indeed, without too much influence from style. Many deliveries possess surprises for the batsman, but a crucial element of cricket, probably the majority element, is making decisions. A good length is decided by the point at which it is most difficult to decide whether to play forward or back and the ideal line is the one that probably won't hit the stumps if it goes straight on, but probably would if it comes back off the pitch.
Creating indecision is vital to beating an opponent. A trained batsman can play swing and spin virtually by muscle memory. At the top level, batsmen are trained to pick up a delivery out of the hand, observing the grip and the action. They are very good at predicting the delivery from that point, allowing them to make an early decision to play or leave. A bowler's job is to push batsmen to make their decisions late, or extract a penalty for making a hasty choice.