I have a very tame driving style in a lot of the cars, Generally I don't have problems, even with the RAC. In fact I love the RAC's twitchy behavior, makes for a fun car to drive, kind of like a Dodge viper, make a mistake and it lets you know.
Difficult Track/Car combo for me? I would say the XRG on any track, its just too flipping slow, and I can't pace myself to it, so I undershoot the corners constantly and it kills my lap time. It's just generally not fun for me. For some reason I have an easier time driving the UF1000.