Mostly just the factory in our cars, as many have said, they are pretty good now, and much of the car's computer system is run through the head unit, so it can be a pain to change. But....
I have a classic convertible that has a massive system to make up for the poor speaker placement options and huge open area. Just went with power, didn't really concern myself with quality just trying to overcome the awesome engine noise (Pontiac 455) and open air noise.