I did gmatclub CAT exams and they are the closest to the final exam. If you can finish these tests in time and score well on them you are all set. I did 5 or 6 of them and they were super helpful. The questions are harder than the normal GMAT questions so on the exam you cruise through the easy/medium questions.
The feature that helped me the most was going through exam solutions. Gmatclub solve's the questions in the most efficient way and it is just very helpful to learn how to solve questions in general for the same topic