I am a big fan of the Spoon S2000. You can't do much to it but it will hold it's own aganst cars with 100 more HP.
I will also fool with R32 Skylines a lot. I must have one of each type of R32 in my GT 2 garage. 4WD, FR, Turbos, NA's, GT-R, non-GT-R. They are all great tuner cars.
Another good tuner car is the Nismo 270R.