I'd say that for me, it's a VERY close call between the 997TT and the 997 GT3. I've driven both and both are fun for different reasons. Believe me, I've driven them hard under ideal conditions.

I also had a chance to try the Boxster/Cayman out at the Porsche driving school. I'd say that both could be "fun" too but its really a personal taste. And this "personal taste" to me means that either the 997TT or the GT3 comes into play. To me, the "fun factor" gets elevated when given the opportunity to drive a car hard through a series of twisties. Power levels, handling abilities and overall feel all adds up to the fun factor for me.