How is Watson presented as the perfect crime partner?

In some ways, Sherlock Holmes is a solitary figure, capable of almost miraculous feats of detection based on encyclopedic knowledge of relevant information, strong observational skills, and a brilliant analytic mind. Holmes, however, despite understanding a great deal about people's criminal habits, has very little in the way of "people skills," often appearing to lack empathy or even the ability to avoid offending people whose help might be useful. He also often does not see the need to explain himself or make himself understood.
Watson, on the other hand, is a doctor with an excellent bedside manner, good at soothing people who are upset, and he often smoothes over the ruffled feathers left by Holmes's abrupt and eccentric manners. Watson projects an air of solid respectability that people inherently trust. Also, in his writing, he provides publicity for Holmes.

Watson is in many ways the perfect crime-solving partner for Sherlock Holmes because in many ways, the characters function as foils.
Sherlock Holmes's stock in trade is his logical mind and faith only in empiricism and deduction. He tends to be tactless and arrogant in his interactions with clients and others whom he meets in the course of solving the crime. Watson's role in this duo is to try to reign in Holmes's abuse of cocaine and morphine, as he does in the story's opening scene, to raise an eyebrow when Holmes claims to be the "only unofficial detective," and to remind Holmes that Mary Morstan is a person and not just a case. In short, Watson functions to save Holmes from himself. Watson is also intelligent and able to keep up with and sometimes challenge Holmes.
Watson is empathetic, kind, and tactful; in short, he possesses the social skills that Holmes lacks, making their partnership effective. When Watson tells Holmes in the story's conclusion that he and Miss Morstan will marry, Holmes replies "But love is an emotional thing, and whatever is emotional is opposed to that true cold reason which I place above all things." Instead of becoming offended, Watson is able to laugh and show his concern for his friend's fatigue, ensuring that their partnership will continue.