Universities have spent the last decade building luxe dorms, gyms, and dining halls to attract students. But a new survey from Boston University suggests the best students aren’t impressed. What they apparently want instead is hard work. Aside from financial aid, “academic rigor’’ was by far the top priority of high-achieving applicants who completed the survey. Observers say that makes sense: with jobs scarce and tuition sky-high, students may be increasingly focused on how college can improve their prospects, not how much fun it can be.