First you have to consider which is the best under which specs?
by Performance points or Power and Weight?
From the last experience I had winning several races in a championship that used 1150kg and 500 hp Gt500 cars. I found the Lexus Sc430 as the best car. Castrol Supra is also pretty good. HSV and NSX were fast also, but they burned their tyres in our 1 hour races, so they couldn't stand against the Lexus, Supras and GTRs. HSV advantage disappear when racing at fixed hp and weight.
Here are the results, these can gave you a idea of which cars to choose.
Nurburgring GP/F:
Pole: Lexus Denso (me) , Winner: Lexus Denso (me), 2nd place: Calsonic GTR
Indy road course:
Pole: Lexus Petronas , Winner: Lexus Petronas, 2nd place: Lexus Denso
Suzuka (variable weather):
Pole: Lexus Denso (me) , Winner: Lexus Denso (me), 2nd place: Castrol Supra
Laguna Seca:
Pole: Lexus Denso (me) , Winner: Castrol Supra, 2nd place: Calsonic GTR (i got disconnect when leading this race)
Monza Variable Weather:
Pole: Motul GTR , Winner: Calsonic GTR, 2nd place: Lexus Denso (me)
La Sarthe 2009 (24h cycle)
Pole: Lexus Denso (me) , Winner: Lexus Denso (me), 2nd place: Castrol Supra
Fuji GT
Pole: Motul GTR, Winner: Calsonic GTR, 2nd place: Motul GTR (I got disconnected here but my Lexus wasn't fast at this tricky track)