The strongest application is supported by at least one professor from your major department and a second letter from another professor who knows you well, or from your Academic Advisor from the Dean's Office/Advising Center.
We look for references that can address your potential for success working at the Unviersity Learning Center. A letter from a TA who knows you well will likely tell us more than a letter from a professor in a large lecture hall whom you've never spoken to (but, as stated above, the best letters come from professors with whom you have built a relationship).
References should be recent and relevant, and should not be from family members or friends in your social circles. Use your best judgment in selecting your references -- your choice in professional references also reflects upon your candidacy.