GTA IV's cops are the best because (along with EFLC) are the only cops in the GTA series who arrest other peds and place them into cop cars.
In IV, cops no longer spawn out of thin air around you. Cops finally acknowledge crimes caused by pedestrians and not just the player
This really can't be overstated. How are we supposed to take cops seriously, how can their behavior be immersive if they concentrate on the player and nobody else, to the point that their cars appear on top of mountains out of nowhere JUST for the player?
As such, cops stop being actual cops, 'characters' that are supposed to be the enemies and antagonists, and become nothing else but an abstract obstacle and artificial difficulty spikes. You might as well need to solve a puzzle every time you steal a car, that's how much sense it makes.
3D era gets a pass, but to clear all the progress and get back to that system... Now there's a feature central to the whole idea of the series that made no sense to revert.