Chicago:
graduate-level classes in economics or related fields, or have had some other significant exposure to research in economics. Many strong applicants have ranked at or near the top of their graduating class.
Havard:
demonstrating the ability to do research is an important criterion.
Increasingly, successful applicants will have taken more mathematics. In particular, most successful applicants now take REAL ANALYSIS, although that is not a requirement.
MIT
Many have taken REAL ANALYSIS or some other advanced proof-oriented course
Minnesota
economic theory beyond the intermediate undergraduate level.  advanced economic theory, mathematical economics, game theory or mathematics for economists.
Courses in REAL ANALYSIS, Differential Equations, Optimization, Topology, Probability or Measurement Theory are all particularly helpful in preparing for graduate study.
Stanford
REAL ANALYSIS is an especially important class because it tends to be demanding everywhere, and forces you to do logical and formal proofs.