The first thing is to define what is a good service.

The service effectiveness is decided largely by three factors: spin, speed and place. An effective service is produced by perfect combination of the three. E.g. a fast topspin or a slow backspin to opponent's far side etc. When opponent's attention is attracted to far side, a no-spin long ball may be served to his close-to-body, e.g. straight into him. These are all effective. Timing is another important factor. You may change the height of toss to readjust height of service.

In addition, the service may be delivered from backhand or forehand, from mid/right/left of table. Combination of these variants produces surprise to cause reception difficulties for your opponents.

Surprise is primarily what we are after. A service may look devastating on itself however, if it's well anticipated by opponent, then it loses all its effect and even creates scoring opportunity for him. A plain service unexpected may render all the result you need. Surprise also refers to a service your opponent has little knowledge of.

You may not be terribly good at service technically though, your service is effective if you know your opponent's weakness and target it precisely.

Therefore, we say service effectiveness is a relative term rather than absolute.

The amount of spin can be measured in many ways. E.g. how much a backspun ball moves back after bouncing on the other side of table or floor, or the deflecting angle when a sidespun ball meets a wall.