The GMAT Club Tests is one of the best. Its pools the questions from various test prep materials and its own. The real beauty is that the answer explanations for the questions, it is not just one explanation but may users given their best short-cut methods to get the answer. That's part i like the most. In additions, the quantitative tests questions are very strong and its provide you with difficulty level of questions where you went wrong in answers.
And the negative part about the tests is the verbal section. some of the questions and answers were not stranded of GMAT test.