I have several favorites that vary far and wide but I have to say that the car I enjoy driving the most, the one that gives me most pleasure, is the Lamborghini Murcielago LP670-4 SuperVeloce.
Aside from the fact that this car was modeled very well (in how it runs), it is beautiful (especially in Bianco Isis & Nero Aldebaran), is a well balanced machine on the circuit, is an excellent workhorse for several expert events, and to top it off with a ripe cherry, it has one of the more beautiful and realistic sounds of all the cars in the line-up (stock muffler extremely necessary!). It gives you that inexplicable satisfaction when you get on the throttle and push its finely tuned V12 to redline.
Add a torque sensing differential set to 10/90 F/R distribution and you have a machine that will demand skill and reward you with pure pleasure as you finesse this noble beast to the fine lines of the limit.