I was asked that question over lunch while visiting the PPE program at UNC, and my answer was this.
In general the market in ideas and reputations of economists works fairly well, at least in the United States. Nonetheless at any point in time, the most overrated economists are the most highly rated young empirical economists at the top schools.
Think of it this way. The half-life of a good empirical result is getting progressively shorter. Good empirical papers no longer stand as definitive accounts for fifteen years and sometimes not even for fifteen months. The science is getting better, but the individual economist is becoming less important, as we might expect from a growing division of labor. That is healthy, but it has implications for the distribution of reputation.
The total amount of repute and renown accorded to individual top young economists does not decline at the same rate that individual contributions become less important. That total amount of repute and renown at say Harvard, available to be doled out to the latest hot young economist, is fixed in the short run or may even be rising, due to the high returns on the school’s endowment.
So those economists end up individually overrated, even though as a whole they become more impressive over time.
Working backwards, one might be inclined to think old theorists and economists who have invented or fleshed out general methods are the most underrated.