For a good car to learn with, you really can't beat the XFG, it's not a tricky car to learn, but you can work on mastering it forever.
I think the most pure car to learn with has to either be the XRG or the LX4, maybe the LX6. These cars will teach you so much, if you can drive the LX6 fast, you can drive anything really.
I really like learning the XRG though, I find it to be just perfectly balanced, XRG @ FE4 is just heaven for me.
The LX cars are pure and balanced too, just faster, and not quite as forgiving. The LX6 is beautifully balanced, but things just happen so fast, it can get out of hand. Great fun though