The best practice CATs are first two free and the second two paid CATs on the GMAT Prep software that you can download from MBA.com after you have created an account.
After that, one option is the predecessor to GMAT Prep, called PowerPrep, which can be downloaded from here.
https://www.beatthegmat.com/mba/resources There are a few issues with PowerPrep though. It does not include an IR section, the SC questions are somewhat out of date, and there is crossover between PowerPrep and the questions in the Official Guides. So if you have used the Official Guides much, if you use PowerPrep many of the questions will not be new to you.
After that come the tests made by Veritas and Manhattan Prep. There is some debate as to which of these are better. I personally prefer some things about the Veritas tests, and for many, but not all, people the Veritas tests seem to generate the most accurate scores after GMAT Prep. The Manhattan Prep tests are known for generating scores slightly lower than those generated by the actual GMAT. Fortunately both Veritas and Manhattan Prep offer a first test free. So you can see for yourself which you like better. Also, at the moment, if you take a GMAT Prep test and send the score to Veritas, they may give you a set of seven tests for free as they are seeking score data to use for tweaking their tests to maximize accuracy of scores generated.