Question

What are the top firms for economists in the US?

Answer 1

Google, or other high tech firm

Answer 2

The FRB for sure!

Answer 3

It depends what you mean by "top".  If you mean where can economists make the most money putting their knowledge to use, while still being recognized by their colleagues as professional economists, then the answer is the financial industry -- without a doubt.  Experienced economists on Wall Street with a superior track record in forecasting business cycles, swings in interest rates, currencies or commodity prices, or in spotting risks and opportunities in financial markets, other industries, or foreign countries, can earn well into six figures and even more than a million dollars/year in salary plus bonus.  As in other situations, the more consistently profitable is the bank, money manager, broker/dealer, or other financial intermediary, the better the chances of earning high figures year after year.  Of course, "there is no free lunch." The work is intense, risky and of a 24/7 nature.

Answer 4

With perfect timing for your question, Athey and Luca just posted a working paper on the topic:
https://papers.ssrn.com/sol3/papers.cfm?abstract_id=3247794

Abstract:
As technology platforms have created new markets and new ways of acquiring information, economists have come to play an increasingly central role in tech companies – tackling problems such as platform design, strategy, pricing, and policy. Over the past five years, hundreds of PhD economists have accepted positions in the technology sector. In this paper, we explore the skills that PhD economists apply in tech companies, the companies that hire them, the types of problems that economists are currently working on, and the areas of academic research that have emerged in relation to these problems.