I think it is worth bearing in mind that the "best" tutoring centre / tutor is going to depend on the needs of the individual student. Group or one-to-one or some combination of the two? Going through theory or answering specific questions? Practice of lots of questions or more selective questions, perhaps aimed at extending skills? Developing the skills to answer questions correctly / accurately or concentrating on exam skills / strategy when understanding is good? A one-size-fits-all model is only effective if the student happens to match the prototype used for the "all".