Santa prefers children who are well-behaved, who give him a lot of milk and cookies, and who don’t ask for a lot of presents. When comparing two children, he will prefer the child who is “better” (better behaved, more milk and cookies, or fewer presents) than the other in more areas.
Dasher is very well-behaved, gives a few milk and cookies, and asks for a lot of presents. Donner has moderately behaved (not well, but not rotten), gives no milk or cookies, but asks for no presents. Comet is very ill-behaved, gives a lot of milk and cookies, and asks for some presents.