Hard to pick just one. Old GP cars, like the Honda RA300 or the Alfa 158, are incredibly satisfying, but also very skill-intensive. The Oreca FLM09, returning from the FM4 days, is almost addictive. Some of the GT cars, like the Emil Frey Jaguar, are also a joy to drive. Group C and Group 5 cars will require some finesse, but they feel mighty fast - as they should. And the list goes on and on.... But if you want two stout, reliable, easy-driven performers, look no further than the classics: the Mazda MX5 is unsurprisingly the most bang that you can get for the least buck: the Super20 is something special, but any of its incarnations will be fun. Anything in the Vintage SpoCom division will also be exhilarating to drive in homologated trim.
Truth to be told, it'd be easier to say which cars aren't fun to drive: it'd be a very short list indeed.