DAT Bootcamp and DAT Genius are the best both in terms of quality and as a learning material. I'm using them both, and they work well together. They are the least expensive and are pretty much all you need for practice tests, in addition to destroyer to round out your learning.
Both Bootcamp and Genius are harder than the actual DAT but will prepare you better by doing so. Use the ADA 2009 DAT to get a flavor for difficulty
**edited to change ADA exam to 2009 as the representative test (had 2007 and 2009 mixed up)