Tyler points to Malthus as a much underappreciated economist. John Cassidy points to Pigou. For my money, Irving Fisher dominates. Other people (e.g. London Banker and Yves Smith)

have also extolled Irving Fisher, but I would still rank

Fisher as highly underappreciated relative to insight and clarity of thought.

Here from his classic, The Debt-Deflation Theory of Great Depressions, are some choice insights.

Then we may deduce the following chain of consequences in nine links: (1) Debt liquidation leads to distress selling and to (2) Contraction of deposit currency, as bank loans are paid off, and to a slowing down of velocity of circulation. This contraction of deposits and of their velocity, precipitated by distress selling, causes (3) A fall in the level of prices, in other words, a swelling of the dollar. Assuming, as above stated, that this fall of prices is not interfered with by reflation or otherwise, there must be (4) A still greater fall in the net worths of business, precipitating bankruptcies and (5) A like fall in profits, which in a "capitalistic," that is, a private-profit society, leads the concerns which are running at a loss to make (6) A reduction in output, in trade and in employment of labor. These losses, bankruptcies and unemployment, lead to (7) Hoarding and slowing down still more the velocity of circulation.

The above eight changes cause (9) Complicated disturbances in the rates of interest, in particular, a fall in the nominal, or money, rates and a rise in the real, or commodity, rates of interest.

Evidently debt and deflation go far toward explaining a great mass of phenomena in a very simple logical way.

With perhaps the qualification that even real rates of interest may fall is this not a brilliant summary of current events? And on the solution to the crisis:

On the other hand, if the foregoing analysis is correct, it is always economically possible to stop or prevent such a depression simply by reflating the price level up to the average level at which outstanding debts were contracted by existing debtors and assumed by existing creditors, and then maintaining that level unchanged.

With a few changes to growth rates rather than levels is this not fully modern? And the following, with its hint of the importance of expectations, strikes a very Sumnerian tone (or rather, of course, Sumner's analysis strikes a very Fisherian tone).

…The fact that immediate reversal of deflation is easily achieved by the use, or even the prospect of use, of appropriate instrumentalities has just been demonstrated by President Roosevelt. Note Charts VII and VIII.

And behavioral economics was nothing new to Irving Fisher:

The public psychology of going into debt for gain passes through several more or less distinct phases: (a) the lure of big prospective dividends or gains in income in the remote future; (b) the hope of selling at a profit, and realising a capital gain in the immediate future; (c) the vogue of reckless promotions, taking advantage of the habituation of the public to great expectations; (d) the development of downright fraud, imposing on a public which had grown credulous and gullible.