Depends how much you want to spend. If you want to stay single exhaust I recommend the MAC axleback. I loved mine when I decided to upgrade but didn't know what to do or spend too much. Best $200 I spent in the beginning plus a simple install.
Afterwards I upgraded again to true duals and that is a whole different story because they cost more and I had to pay for installation since it was too advanced for me. But in the end duals are better for looks, sound and performance.