**Yes and no**, while the $20 note is the most commonly encountered counterfeit bill in the United States, the $100 note is the most common counter fit bill in worldwide circulation. 

It appears that the $20 note is typically the bill most likely to be attributed to being counterfeited in [various factoids][1] without that distinction. However, in journalistic articles that feature quotes attributed to representatives from the [United States Treasury Department][2] and the [United States Secret Service][3] the $100 note is cited as the [most][4] [commonly][5] [counterfeited][6] in world wide circulation. 

The worldwide circulation is the interesting aspect of this answer as there are a number of different techniques that are used by well-funded organizations for counterfeiting (from bleaching US$5 bills to turn into $100 note to making new ones with stolen engravings) that tend to provide indications as to where the bills are being produced.

Counterfeiting is a big enough issue that the [Federal Reserve][7] periodically releases reports about the problem. In a report issued in [January 2000][8], one of the key findings was as follows:

> ... the denomination with the largest amount of counterfeits, both in
> dollar terms (about $6 million) and as a proportion of notes processed
> (about 60 per million notes), was the $100 note.<sup>1</sup>

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1. See page 52 - 53 which includes a table summary of counterfeiting rates at Federal Reserve banks.


  [1]: http://www.fuzeqna.com/ccul/consumer/kbdetail.asp?kbid=362&ap=t
  [2]: http://en.wikipedia.org/wiki/United_States_Department_of_the_Treasury
  [3]: http://en.wikipedia.org/wiki/United_States_Secret_Service
  [4]: http://www.reuters.com/article/2010/04/21/usa-currency-idUSN2119469720100421
  [5]: http://business.time.com/2012/02/24/how-the-u-s-could-pressure-north-korea-tomorrow-quit-the-100-bill/
  [6]: http://www.csmonitor.com/2007/0920/p14s02-wmgn.html
  [7]: http://en.wikipedia.org/wiki/Federal_Reserve_System
  [8]: http://www.federalreserve.gov/boarddocs/rptcongress/counterfeit.pdf