I guess you don't have a X1 nor a Furai.
If you have the money, try to buy the McLaren F1 and tune it to the limit. It will be a good race car but also a tough battle.
Or you even grind more till you can buy the 908 and you will eat the competition for breakfast.
I also guess you won't want to use any birthday glitch to get those cars, otherwise you wouldn't ask.
The Enzo is hard to handle, Lambos suck, maybe the SV but it will be hard as well. R8 V10 to heavy. Super GT Cars a little bit to weak.
Race Modded Corvette will be also a hard battle.
I suggest you need to grind in B Spec to get/win some better cars.