Brad DeLong says "nobody":
Who Can Replace Larry Summers?, by Brad DeLong: My view: nobody. The Obama administration is going to be weaker as a result of his forthcoming departure.
The job of Assistant to the President for Economic Policy has two components:
Making sure that the President hears and considers the arguments, and makes an informed decision.
Making sure that everybody thinks that the president has heard and considered their arguments, and made an informed decision, and that they are valued and respected members of the team who are being given due influence and deference.--and also to make sure that the president hears everybody's arguments and makes an informed decision.
Larry is superb at figuring out how to present complicated arguments to make them comprehensible to non-experts, and at setting up frameworks for discussion and debate that kept policy discussions on track and organized. That is most of the job. That is (1). And at that Larry is the best in the world.
The other part, part (2)--making sure that everybody thinks that their point of view has been given a fair shake, and that they are valued and respected measures of the team? Well, given who Larry is, he far, far exceeded expectations. Only one public shouting fight in the halls of the West Wing in two years is... quite good, really.
If I were Obama, I would (a) move Tim Geithner over to the West Wing job, for that is the job that matches his skill set where I think he would indeed by best in the world, and (b) bring Laura Tyson in to be Treasury Secretary...
Failing that, were I Obama, I would probably try to grab Alan Blinder from Princeton to fill the West Wing job...
And were I really devious, I would bring Ben Bernanke over to the West Wing and put Larry in the Fed Chair job...
That he wasn't as bad as expected at half his job "given who Larry is" doesn't say that he did it well, and it doesn't say he did it better than anyone else would have. I don't see how that translates to "irreplaceable." The public perception is that he captured the White House conversation and directed it in a particular way, and at times prevented dissenting views from being fully expressed. True or not, public perception matters and the idea that Summers was the one standing in the way of more populist policy was commonly held and hurt the president's standing with the base.
Maybe he's so good at the first part of the job that it more than compensates for failures at the second, but that depends upon the weights you attach to each part of the job, and it also depends upon the assumption that nobody else would be better at the first part. I guess I place more emphasis on the second part of the job than Brad, and I very much doubt that Summers is the only one in the world who can effectively "present complicated arguments to make them comprehensible to non-experts, and at setting up frameworks for discussion and debate that kept policy discussions on track and organized." There are others who can do this well, I think just as well. But even if they aren't quite as good at the first part of the job the difference wouldn't be that large, and their ability to do better at the second part of the job would more than compensate. Summers is not irreplaceable.