MBTI or Socionics?

Which theory is the most accurate when it comes to best matches? MBTI or Socionics?

I wouldn't say that one theory is necessarily more accurate than the other regarding relationships. I do think the socionics theory emphasis that aspect more though.

MBTI has no theory of matches. It has been developed as a career research tool really for people to discover their own strengths and weaknesses and make a better decision about their college major or the next job: "The original developers of the personality inventory were Katharine Cook Briggs and her daughter, Isabel Briggs Myers. They began creating the indicator during World War II, believing that a knowledge of personality preferences would help women who were entering the industrial workforce for the first time to identify the sort of war-time jobs where they would be "most comfortable and effective". -- Reference: Myers-Briggs Type Indicator

So MBTI doesn't go into inter-type relations. It only emphases each type's strengths and weak areas, especially translating them to the workplace. Socionics does go into inter-type relations, but it is sort of too theoretical. Real life relationships are much more complex than interaction of 8 cognitive functions really. Plus some of the socionics terms are a bit misleading. For example you can be friends with a person who is your conflictor in socionics. Chances are you won't understand each other very deeply but it being a "conflictor" relation doesn't actually mean that you're actually going to start fighting. Similarly, duality relations which are considered to be the best do end up with break-ups and people being disappointed with each other. So take it with a gain of salt.