Yes and no, while the US$20 bill is the most commonly encountered counterfeit bill in the United States, the US$100 bill is the most common counter fit bill in worldwide circulation.
It appears that the US$20 bill is typically the bill most likely to be attributed to being counterfeited in various factoids without that distinction. However, in journalistic articles that feature quotes attributed to representatives from the United States Treasury Department and the United States Secret Service the US$100 bill is cited as the most commonly counterfeited in world wide circulation.
The worldwide circulation is the interesting aspect of this question as there are a number of different techniques that are used well funded organizations for counterfeiting (from bleaching US$5 bills to turn into US$100 bills to making new ones stolen engravings) that tend to provide indications as to where the bills are being produced.