haha, well, I've only used the GMATPrep, PowerPrep,
MGMAT, and Princeton Review. If you add Kaplan and what I've *read* about it, here would be my rankings:
1. GMATPrep - by far the most representative of the GMAT in terms of both scoring and test material
2. PowerPrep - Also uses real test material, but the scoring is a bit more lenient. The math questions are much easier, and the verbal is roughly the same. The only problem is it repeats a lot of questions from OG10 and OG11.
3.
MGMAT - Uber-hard math will help you learn tricks to speed up your math and deal with tricky questions. Verbal has some problems, but overall is similar to GMAT, but slightly harder. The quant score is roughly 2-4 pts lower than what you could get on the real GMAT (assuming you don't get nervous or freeze up). The verbal score is pretty spot on. This provides the BEST comprehensive review of your test and the answers. If anything, I would take the test just for that!
4. Princeton Review - The math is extremely easy, the verbal has so many "wrong" answers and awkward RC/CR passages. I really didn't think this test would help me at all on the real GMAT after taking it once. The answer explanations for math and verbal were both not too good.
5. Kaplan - I've never taken it, but it's only because EVERYONE here says the score is roughly 100 pts lower than your potential real score, and the verbal and math were both unrepresentative.
I hope that helps!