One of my favorite things to do is to start a fight with someone and then run towards a cop. It's so entertaining watching them throw a punch only for them to put their hands up in surrender when the cops draw their guns, and make the walk of shame back to the car. Sometimes, I even like to follow the car around.
Gang members are the best since all you have to do is bump into them. Plus, whenever I get a gang member arrested, I feel like I'm helping the community with each gang banger off the street.
The cops in GTA V and GTA Online don't seem to arrest people and seem to have more of a shoot first mentality. Maybe they do, but I've never seen it. This is one of those little things that makes GTA IV still oh so engaging.