The GMATPrep tests
are the best ones, but I don't know that I'd really use it for a diagnostic. I wouldn't waste those tests before you get into your prep. Doing a diagnostic cold, without knowing anything about the GMAT or doing any practice questions first, wastes a test for most people who aren't naturally gifted test-takers.
Now if you've already done some practice, then I'd go for it!
The other possibility is to do the free diagnostic test on http://www.review.com
. Princeton Review
's tests are very similar to the real thing, so it would be a good diagnostic, although I don't know if the one on their site is also on the DVD that comes with Cracking the GMAT