2007 Audi S6 quattro 4dr Sedan AWD (5.2L 10cyl 6A)
This car has been fantastic over the past 2 years! The only faults are that it is heavy car, and isn't as nimble as my 3 series BMW. However, the front seats are amazingly comfortable, the interior is top notch, and the power and torque are monstrous but precise...fantastic engine growl and acceleration. No denying that a V10 is truly addictive. Gas mileage in non-sport mode is quite good...around low 20's mpg. The only issues I had have been electrical...automatic headlights didn't function correctly after a few months and had to be completely replaced (under warranty). Also, I have gone through several sets of tires prematurely...you have to keep an eye out for uneven wear due to weight.
Style, quality, performance, handling, bluetooth, nav...the best has to be exclusivity and its stealthiness. It's a wolf in sheep's clothing for 4. It's a sleeper that gets unfairly compared to the M5 which costs about $20-30k more.
Make it lighter!! Put a carbon fiber hood and roof on this car. Get it under 4000 lbs. Update the front end to look more aggressive like the S5. Move the buttons away from the cup holder (it catches spills, then locks up the buttons). Simplify the buttons: too many unnecessary around the MMI knob.