Now a days the company is providing its own sound system. It is enough for the normal sound. If you want to additional extra sound then you want to add other sound system from outside.
Some of the good sound system is PAudio, BOSS, JBL, Artis, Pioneer.All of them are good for the professional sound which is far enough to your car. Depend on the dimension of your car the sound system is applied. for example if we have 4 seat car llike swift or wegonar then 2.1 channel system is enough.
There is 5.1 channel system is also available for the audio and some of the system comes with the builtin amplifier which is applicable for suv cars.
The drawback of these additional outside sound system is if in case any problem is happen with your car regarding electric cables. then the car company is not able to help you to resolve the same.