I took my test on March 12th, after going through more than 15 CATs. Undoubtedly, the GMATPrep ones were the best. Additionally, I tried various test like Arco, VeritasPrep, 800score etc. All of these tests have some or other drawbacks. The only drawback I found on the GMATPrep tests was that they did not have any explanations for the answers. Still, given that they represent the test most accurately, trust them.
My suggestion, take as many tests as you can, from various sources, for the sake of practice, however, for score assessment, only trust the GMATPrep ones. You may read one of my posts at
https://www.beatthegmat.com/my-debrief-f ... 54581.html, which has a detailed description of my experience with these online exams.
Hope this helps.