These are probably my current favourites.
Maserati Gran Turismo S (fully upgraded, including aero upgrades and racing tires. Gorgeous car, even with a big spoiler and carbon fiber bonnet. Sounded like music to my ears before upgrades, now it just sounds brutal
)
Ferrari F40 (no upgrades. Just a joy to drive and it sounds great)
Aston Martin DB9 (no upgrades. Not insanely fast, but also great to drive. Again wonderful engine sound, and I love the sky and overhead objects reflecting off the large center console in cockpit view)
Mercedes C63 AMG (no upgrades yet. Not a very pretty car, but I love the sound and the handling. Unless I needed something really fast I would pick this over the Merc SLS any time)
Lexus LFA (not upgraded. A blast to drive, sounds great, looks mean and has a pretty cool dashboard)
Zonda R or McLaren F1 stealth (no upgrades yet. For when I need to go
really fast. And both are great to drive as well)
Audi R8 V10 chrome (upgraded to 835 bhp. Actually I've grown a bit bored with this car, but I loved it during my first many hours of a-spec, and my b-spec drivers are still using it frequently)