Just some of my favs.
Amuse:
Carbon R (only in GT mode)
S2000 R1 (best of Amuse IMO)
S2000 Street
Aston Martin:
DB9
Audi:
R8 (my favorite A-class super sports car)
BMW:
M3 '04
M3 CSL
McLaren F1 (most rewarding lap)
Chevy:
Z06 (C5) (one of my personal favs for the underdog role)
Honda:
NSX Type R '02
NSX Type S Zero '99
Jaguar:
XJ220 Road Car
Lotus:
Elise Motor Sport
Elise 190
Elise 111R & 111S
Mazda:
RX7 LM
Mitsubishi:
Evo 8 GSR & GSR MR
Evo 5 GSR
Nismo:
R-tune R34
Z-tune Z33
Nissan:
R34 (almost any model)
Xanavi Nismo '03
RUF:
RGT
BTR
3400S (most balanced of the RUFs in my opinion)
Subaru:
WRX Sti '02 (type II)
Toyota:
TRD Celica
That about wraps it up for me.
Of course, there are dozens of cars I am leaving out.
With so many race cars in the game it wouldn't be hard to go on for a long, LONG, time with a list of race cars I enjoy on the Ring.
Needless to say though...
Those cars listed above are some of my favorites.