It all depends what you're after...
If you want some more noise and you're not too bothered about power then a Cat-Back would be the best option. As mentioned above, you could also have the pre-cat removed in the OEM downpipe along with this.
However if you want power, then a 3" downpipe with sports cat or decat is what you will need but you wont have as much noise as what you may be looking for if thats what you're after