It really depends on the price range you're looking at.
Personally, I have a Motordyne Shockwave CBE. IMO second best sounding exhaust under the ArmyTrix one. They run a little under or just about $2,000 USD for a new one with burnt exhaust tips. My Z is my daily driver and at cruising speeds it sounds fine. No drone and not terribly loud. Once you WOT though, it screams. As far as performance goes, (with a tune) you'll see about 15-20 hp gains.
Here is a decent write up of the Motordyne Shockwave exhaust:
Review: Motordyne Shockwave))) E370 Exhaust