I chose Discover, because at the time they were the only ones that I could find without any origination or other fees. Ask your financial aid office at your school what things you ought to look for.
In the future it will be the guarantor and not the lender that you will be dealing with. So, you may want to call the guaruntor and see how easy it is to get a hold of a person, and how well they can answer questions.
So for the Stafford loans, most lenders are the same rate (6.8% just differences in fees), and for GradPlus loans the rates are higher. Private loans (to supplement the Stafford loans) vary in rates, so shop around more to find the best lender. What are the rates for private student loans nowadays, or where is a good place to check them?