what is the assault weapon of choice for US police forces?

The term "assault weapon" means nothing in terms of weapons

however assault rifle does- an assault rifle is a select fire rifle

that fires a round in between the size of a full sized rifle and pistol round.

The weapon was developed by the Germans in World War II because the average infantry engagement took place with in 300 yards.

Sub Machine guns had about 100 yard range and a high powered rifle like the Mauser was cappable of taking out targets at 1000 yards They developed the MP44 which was then named Stormgeweir or Assault Rifle.

The only police officers in the US who carry assault rifles on a regular basis are the SWAT teams

and the photos I have seen- the assault rifle of choice is the M4.

They also like sub machine guns like the MP5 which is more common than assault rifles due to their shorter range.

Generally they are reserved for SWAT teams. Most use Military grade M-16's of various models. I've see seen others with the H&K G36 & MP5 sub guns. The Springfield Armory M1A has fans too in short barrel models.

Most local and city SWAT Teams would likely use variants of the M16 (M4, AR15, etc.). Some elite federal teams like the FBI's Hostage Rescue Team may use weapons like the HK416.

There is over 10,000 police departments across the US, and they all issue different weapons.