Best Test Series for GMAT

Gmat Club test series is undoubtedly the best test series that is out there.
The quant part of the CAT have the best questions that you will find anywhere. If you want to score 700+, this series is a must have and it's not just the questions themselves, the explanations provided also help you uncover many shortcuts and help strengthen your core concepts. Admittedly, I found them to be a little harder than the GMAT quant questions, but that should help you be better prepared for the final test. And also, you can take the quant section or verbal section one at a time, which gives great flexibility when you are hard pressed for time.
The quality of verbal questions is okay.
I used my GmatClub tests along with MGMA tests to practice the CATs, and I will suggest the same strategy to others appearing for the GMAT.