Gen 3 G17 is the most issued service pistol on the planet and is in heavy use by most of the tier 1 spec ops units.
USP 9 is the next most issued, and may eventually be replaced with the P30. HK builds very solid pistols with questionable triggers IMO.
SIG P226 is a great pistol but their commercial stuff has suffered since production moved to the US and old QC standards were abandoned. Still in use with a lot of heavy outfits.
I would take anything off this list, particularly if I could restrict the 226 to West German models (which is exactly what I have done here at home, where my wife keeps a German 226.)
Third most issued pistol is the CZ75...issued almost exclusively in third world countries, though.