I really don't have any favorite car singled out so I'm gonna lay out my top 5:
1. Toyota Castrol TOM'S SUPRA '99
~ My favorite car from the first game up to this, it's sort of a racing icon for me who has the Supra for his favorite JDM car. Even though the '01 model handles better than this, this car just takes the cake for me.
2. WEDSSPORT Celica
~ I once looked at this car and thought it was just like the regular GT300 cars, something that has power close to stock and just had better handling, but this one blew my mind when I took it for a spin in the Nurburgring. Fantastic car to drive, probably the best handling GT300 car in the game, better than the ASPARADRINK imo.
3. Mercedes SLR McLaren
~ I never really appreciated the amazing capabilities of this car until Week 87 of the WTAC, this car is one of the best in its class. You can slap on S3 tires on this car and it provides you with a fun yet challenging drive. Fast and fun, what more could there be?
4. Amuse S2300 GT1
~ I have to say, the only thing keeping this car from being on par with the JGTC NSX cars, is a JGTC NSX engine. Though the power of this model may be a bit too much for what originally was a 2-litre engine, how the car attacks the corners is what makes it superior to most of the tuner cars.
5. Bentley Speed 8
~ I'm an avid fan of the LMP series cars, and seeing the iconic Speed 8 is just a joy. I've driven the car in one of WTAC's challenges at R1 tires and it's such a fast and agile car. I won't really wonder why Audi's R8 LMP car had such a hard time against this car in the 24 Heures du Mans.
As for least favorites, I don't have any. Every car has something likeable about it in this game, even though I just bought most of them for collection's sake.