map

MAP is MLE plus a prior, and that prior is not academic decoration: it is the same thing as regularization, and it is what stops a model from making confident nonsense out of three data points. The connection that makes this an applied answer, not a stats-class recitation.

Accuracy is meaningless for ranking, the whole point is the order. The real question behind the question is whether your relevance is graded or binary and whether one good result is enough, because that picks the metric. Then the trap: your offline NDCG can climb while online engagement falls.