"The GMATClub tests
are the hardest practice tests available."
I started my gmat prep here, at the gmatclub, and so I decided to see how I would do on the diagnostic questions. I was just shocked and appalled by how difficult I found the questions and wondered if I could ever study enough to be prepared for the math portion. It was quite a relief to find out that the questions were designed to be difficult -- not a good representation of the actual exam. (Although, I haven't taken the real exam so it could be just as bad)
I like it harder anyway. Difficult questions stimulate the mind and forces recall of even the minute details of specific math fundamentals (and of advance math topics). Based on my research, the GMATClub questions are great for those who aim for very high math scores. (90+ percentile)
I've also taken one of Knewton's tests and I feel that their questions are more tricky than difficult.