When a functional works well, Jacob’s ladder does not discard it but builds upon it. In that sense there are 5 ‘rungs’, where the first one is the LDA (electron density), the second GGA (gradient of density) and the third meta-GGA (kinetic energy) which is still in development (note: the article was from 2002 so maybe this has changed now). The fourth rung would be an exact determination of the exchange and the fifth is solving it exactly for a part. When this is done one would expect to get a ‘divine’ functional which is exact and can exactly solve the system.