According to Ravin Thambapillai's analysis, exporters are the biggest winners, so I looked up some World Bank export data.
From
http://data.worldbank.org/indicator/NE.EXP.GNFS.ZSBiggest exporting country zones (% GDP)
41% Europe and central asia
45% Middle east and north africa
42% Small States (population <1.5m)
http://data.worldbank.org/region/SSTLowest exporting countries (% GDP)
13% USA
12% Brazil
15% Japan
14% West bank and gaza
11% Rwanda
9% Nepal
12% Pakistan
13% Haiti
12% Ethiopia
Countries with exports over 100% of GDP:
209% Singapore
165% Luxembourg
112% Macao
223% HK
101% Ireland
Countries with exports between 75% and 100% of GDP:
Belgium
Brunei Darussalam
Congo
Czech Republic
Estonia
Guyana
Hungary
Malaysia
Malta
Netherlands
Palau
Puerto Rico
Seychelles
Slovak Republic
Thailand
UAE
Vietnam
Considering only large economies, Germany, China, and Canada are the top exporters at over 30% of GDP, while the US, Japan, and Brazil are on the bottom. Germany and China have their currencies tied to their biggest trading partners within free trade zones, but Canada floats its currency, even though 84% of its exports go to the US. So it generally appears that where international trade is high, the disadvantages of giving up monetary sovereignty can be outweighed by the advantage of eliminating exchange rate risk, at least within a free trade zone.
Ravin Thambapillai's answer states that the loss of control of monetary policy affects all countries more or less equally. However any country that pegs its currency to another one has already given up monetary sovereignty, so the main downside of a world currency does not exist for these countries. This list includes China, Venezuela, Saudi Arabia, Ecuador, Panama, many Caribbean islands, and many central and west African countries. Smaller countries in the euro zone have also given up monetary sovereignty to a large extent.