That's tough, because what sounds good to someone might sound bad to you...
Do you want loud?
Do you want quiet with better flow than stock?
Do you want just like stock but cheaper?

 

If you don't want any sound, then why do you want an exhaust?

 

What about something just a little louder than stock but cheap?

 

Not many direct-fit aftermarket exhaust options for the N. American 7th gen. The Walker/Bosal replacements do fit but don't look or sound as good as OE. The Yonaka kit is nice but looks somewhat out of place on a 7.5 sedan.

If you want to build your own cat-back system(and your local laws allow it), you could start with this place. While they don't offer complete systems for our cars, they do offer mandrel bent cat-back tubing kits in a variety of options(eg: 2.5"/3", single/dual, aluminized/stainless). You can acquire an appropriately sized resonator and rear muffler in whatever decibel/price level you desire. Of coarse, assembly will require metal cutting and welding but any capable exhaust shop could probably handle it.