Assuming your department will purchase whatever you want, I believe the ideal rifle is a AR-15 with a 14.5 to 16 inch barrel, with flat top for mounting non-magnified red dot optics, backup iron sights, and a collapsible stock. Add to this a sling and flashlight or weapon light. Great for all around patrol use and CQB if pressed into it. Also the red dot optic, such as an Aimpoint, is good for low light and the flashlight is good for suspect ID in low light or darkness. The sling allows for faster transition to the pistol in the rare event the weapon has a catastrophic failure.
As for which one, thats a thread all unto itself.
The top seem to be Colt, Bushmaster, Rock River, Armalite, DMPS.
FWIW I am the department Rangemaster and armorer. I am also a 20 + years LEO.