The Raybrig NSX is a great car. Stock, it will handle the other JGTC cars. With some additional power it will battle some of the upgraded race cars as well. It's probably the most useful of the 4 cars. The GT-40 is the road car version, I believe. It's definitely cool, but not as useful in the game. Either of the two Skylines are good, with the Mine's being more built up. The Mine's Skyline is actually a good car to tackle rally with, if you don't have a rally car.
My first vote would still go with the Raybrig NSX.