For me, part of what makes a car fun to drive isn't just the way it handles, but also the sound of the engine, and the overall appearance of the car. The Lexus LFA with upgraded suspension and tires (you could try driving one without the upgrades) is a car I consider very fun to drive. I just recently purchased one along with a set of other cars to drive around Nurburgring (I never really liked this track in the past) and love this car. It's not the fastest car nor is it necessary the slowest, but the sound of the LFA's V10 along with a rwd drivetrain make this car a very fun one to drive.
As far as other fun cars to drive, I don't think you can go wrong with rwd cars such as the old muscle cars, Ferrari SP1, Lotus Elise 111R, etc.