Effective serving is MUCH MORE than the three factors OP listed, but yes they are important. NO WAY to say ONE is the most important.
Serves are connected with your overall strategy and specific tactics. A serve should be used to gain an immediate offensive advantage. There are many ways to do that. There are many ways to take advantage of a good serve.
At some higher levels, most serves do not produce an easy putaway ball or a super clear offensive chance. At that level, it is all about limiting the receivers' options and choosing how to open the offensive.
Spin is important. Especially underspin. I think it is real important to show an opponent real early that you can produce heavy underspin with a very short stroke (that looks the same for each different serve). That makes your SPIN VARIATION much more effective when you can do so making the changed serve look the same.
it is important to control height, even the best short serve is an easy putaway if it is high.
it is important to change up the serves and be unpredictable.
it is important to learn what serves trouble oponents.
it is important to save a good serve for critical points.
it is important to have deceptive follow throughs.
it is important to make your serve motion very smooth and fuid quick, so that it is difficult for hte opponent to know exactly what bat angle and bat speed you used.
it is important to practce serves every chance you get.
it is important to make your short serves stay short, but near endline, your deep serves bounce near endline, and your extreme short serves very short
it is important to understand what serve will be returned in what manner likely, so you can be prepared to open the attack first.
it is important to have a PLAN for what you want to do on serve and ways ot cope if you get a different ball back.
it is important to be LOOSE with grip and arm.
it is important ot NOT use upper arm so much.
I could go on, but you see how many important aspects of serves I though up in just 5 minutes of typing? This is to illustrate that there are SO MANY things to serving we cannot just look at a few and call it a day.