First, as someone said earlier with assassinate, you need to score a hit.
Second, in order to score a critical, you have to have an attack roll. See attacks and rolling a 1 or 20.
Third, the only spells that can score a critical are the ones you make attack rolls with. Confirmed by Mearls, and backed up by attack roll rules.
Fourth, the you have to hit for a condition to give you an autocrit. Again under attacks.
That is RAW. However you want to play/interpret in your game is fine.
You keep saying this but don't back it up with you know rule references.
First, "score" is not a game term and has no meaning in the context of this conversation. Assassinate says "hit", magic missile says "hit", that is the key term here.
Second, in never says you need to have an attack roll to score a critical hit. The section you are talking about mentions how you normally score a critical hit, it doesn't have limiting wording that implies this is the only way to score a critical.
Third, save spells can't crit because they don't hit. The only spell that has the word "hit" in it, that doesn't use an attack roll is magic missile, the designers chose to use that word not me.
Fourth, again it never says that is the only way to score a critical hit.
So you can play by the intent of the rules, which I agree is most likely magic missiles shouldn't ever get a critical. But that is not RAW. Before you say something is RAW maybe you should read the rules, and if you want to back up your point of view maybe a direct quote or two.