Depends on how much time you have. The best CATs out there are IMO are:
GMATPrep
MGMAT
PR
PowerPrep
In order of difficulty, I would rate them (from easiest to hardest) as:
PowerPrep
PR
MGMAT
GMATPrep
If you have a lot of time on your hands, I would recommend going from easiest to hardest so you can get acclimated to the CAT and see progressively harder material as you move forward. No matter what though, be sure to take both GMATPreps. The last one should be taken at the end of your studies since it is the most accurate indicator of performance. I would take the first GMATPrep in the middle of your preparation. The reason is that you want to see how well you perform on the best CAT out there and learn from the problems and investigate your mistakes while you still have ample time to improve. Some people recommend taking the first GMATPrep in the beginning of prep, but I think that's a waste since you only get two official practice CATs. You can gauge where you stand at the beginning of your prep with PowerPrep.
