A lot of departments dictate what ammo is carried on duty, while off duty though most must buy their own and some go their own way.

Handgun ammo for some LEO departments is dictated by the armour. Change the armour and the ammo sometimes goes too. Some places like Colorado Springs uses an ammo made to their specifications for them by Winchester (Olin), others use factory offerings. Speer Gold Dots and Federal Hydra-Shok have been staples for years. Some departments are still carrying Black Talons, yes, not SXP's. Remington Golden Saber as well as Hornady XTP's are also very popular.

With the technology of ammo today, it is truely hard to make a bad choice if you stick to premium ammo.

Here is a link as to what Ayoob thinks:
http://www.backwoodshome.com/articles2/ayoob93.html
It talks about ammo.....

His way of thinking in the past was......carry the largest caliber you feel comfortable shooting accurately, and the heaviest soft tip bullet available. to some that means a .22lr, to some a .500S&W.

Personally what I carry:
.357 Mag 158 grain Hornady XTP
10mm 180 grain Hornady XTP
40 S&W 165 grain Reminton Golden Saber
9mm +P 147 grain Remington Golden Saber
.380 ACP 90 grain Cor-Bon JHP +P

I do not endorse the loads as the best out there, but are proven reliable for function and accuracy in my firearms.