I'm labeling only supercars and performance cars, as I think the Need For Speed series was known for them.
BMW
M3 CSL, M5 (2000 and 2005 models), M3 (E30 E36 E46), Z8, Z4
Mercedes
C-class AMG, SLR, E55 AMG, C43 AMG, SL55 AMG
Porsche
911 (964 993 996 models), 911 Turbo, Carrera GT, 911 RS (new one)
Ferrari
360 Challenge Stradale, 575M, Enzo, F355 F1, 250 GTO, F40, F50, 612 Scaglietti, 1968 365 GT 2+2
Lotus
Elise (new and old), Exige, Esprit, Elan (new)
Lamborghini
Diabo VT, Murcielago, Gallardo
Chevy
Corvette (ZR1 C5 C6 models), Camaro SS, SSR
Pontiac
Trans Am Ram Air(2001 model) GTO
Ford
Mustang Cobra R, Focus RS
Dodge
Neon SRT4, Viper SRT/10
Cadillac
XLR, Cien, CTSV
Nissan
Skyline GTR (R32 R33 R34 models), 350Z, 300ZX
Infiniti
G35 (coupe and sedan)
Mazda
RX7 FD3S, RX8, Miata Mazdaspeed
Toyota
Supra RZ, Celica GT Four RC, MR2 GT-S
Aston Martin
DB7, DB9, Vanquish
Caterham
Super Seven
Noble
Noble M12
That's it really, I might have forgotten some, post em later. I liked Underground, but I miss the Need For Speed I knew and grew up with. If they kept the same physics engine and put back the exotics, I'd be estatic. Not that I don't like import cars and aftermarket tuning (I'm really into that), it's just I wished Need For Speed stayed true to it's roots.