The first RoboCop movie featured the 1986/1987 Taurus LX as all the police cars. You can tell they're LX because they have the multi-spoke wheels (minus center caps) instead of steel wheels and hubcaps; the close up of the car peeling out in the police garage shows the side trim showing "Taurus LX"; and they all have a cornering light in the bumper. One thing that made it stand out was the headlights. If you look carefully, the headlights on the car used in the movie uses round four-eyes behind a clear plastic cover instead of the standard composite headlight and integral turn signal/parking light found on the standard Taurus. Images below are from the first movie.

The second movie used the 1986-1988 Taurus L for most of the fleet. They have black steel wheels and solid red taillights. The headlights are stock Taurus headlight unlike the custom lights in the first movie. Some GLs were used and can be spotted by the amber turn signal in the rear. Below are images from the second movie.

The third movie used the same 1986-1988 Taurus L from the second movie. Some 1986-1988 GLs can be spotted as well. There is also the addition of a couple 1990/1991 Taurus L. In the first chase scene, the same car changes from a 1986-1998 L to a 1990/1991 L to a 1986-1988 GL! Below are images from the third movie.