All the rep ranges will work for you; it just depends on your specific goals. Generally, if you are building muscle 8-12 reps is a good range to work with. But don’t stay in this rigid range forever. If you have been doing this for an extended amount of time, you are not going to continue to get the returns that you are looking for. Try increasing the weight and decreasing the reps. Then you can slowly get back to the 8-12 rep range with the higher weight and then keep it fresh and keep it cycled for continued improvements.