Glock the most, Some departments use Sig like the Michigan State Police, ohio state Highway Patrol, texas Rangers, some homeland security personnel, coast guard, SEALs. Beretta is standard issue to the Army and USMC as far as I understand.
Some agencies in the US like the Atlanta PD is switching from Glock to S&W's M&P line, some agencies offer HK USP as an option or as standard issue.
I know that the BSO (Broward Sheriff's office) here in FL uses Glock, but they allow people to use Sig if they are grandfathered in. What does that mean? I really don't have a clue lol, but that's what I was told when I was looking at Sigs in the shop.
You know what though? It doesn't even matter to me. I mean, if everyone in the country was issued a Springfield XD I would not buy one based on that. I bought what I have because of comfort, quality, reliability reports, and personal taste. Obviously it has to shoot good too.