It's not the number of cops I've put down, but the number of hours I've spent doing it that makes me bashful.
"L-S FUCKING P-D!", or "STEP OUT OF THE VEHICLE, WE NEED TO HAVE A LITTLE CHAT.", cracks me up every time.
My favorite thing to do is make the NPC's interact, and specifically, make them kill each other. I've started a little gang war between construction workers and ambulance drivers. I've chased a dude on a bike and made him pedal down the freeway into oncoming traffic. There was a guy I named "Cactus Pete" because he got stuck in a flowerbed and was resigned to running headfirst into a cactus, stumbling backwards, and running back into the cactus again. A woman stopped to watch for several minutes, but had to be on her way, as it had started raining.
My buddy used to get online too, and we'd come up with all kinds of things to do, but the following stands out. One of us would get in a submarine, and the other would fly the helicopter with the grappling hook. We'd take the submarine up to the top of Mount Chilliad, and see how far down the face of the mountain we could slide it after gently detaching from the grappling hook. The sub would always explode after grating against the mountainside for a time, killing the occupant, but the submarine slide was far from over; it was simply entering the "post-mortem" phase.