You should follow only official questions for GMAT because GMAC has published those questions. You are going to play in the field in GMAC, and GMAC has published their rules via those official questions. Try to digest those official questions. On the other hand, there are lot prep companies, and there are lot of questions. When you will be given all those questions, you may lose yourself in the midst of questions.My advice is to follow official questions only.

1. Official Guide 10th edition
2. Official Guide 11th edition
3. Official Guide 12th edition
4. Verbal Review
5. GMATPREP--> There are lot of questions for practice in GMATPREP.

I think there are more than 700 sc, 500 cr, and 200 rc. if you can digest all those rules tested in those official questions, you will surely crack GMAT.
Thanks.

You could also take the MGMAT practice tests. I thought the math was a little harder than the real test, so it ended up being really good practice.