To me, the ideal fun car is anything that's around the 3000lb, 300HP, RWD mark - give or take.
This is why I love my Z so much. It matches every pound with horsepower. It weighs 3300lbs, and packs 332HP. I will turn blue in the face preaching this. It's one of the reasons why the E46 M3 was such a hoot to drive back in the day (even the EU E36 M3, and the Z4 M Coupe), the balance of weight and power and precision was truly perfect for the car.
That's why the E92 M3, as fantastic of a car as it is, wasn't as highly revered - it was confused, it didn't know if it wanted to be a sports car, a muscle car, or a grand tourer - a common plague that the M-division has been suffering from for the past 4 years now (thanks X5M, X6M). And that's when the 1M Coupe came in and saved the day with a back-to-basics design. I only say this because the E92 M3 was a car I test drove twice, once 6MT another time DCT - and just couldn't figure out what was it about the car that didn't grab me. I loved the car, but I couldn't see myself owning one, as unusual as that may sound. The 335i seemed like a better choice, too (which is why I had my GF get one after she test drove it with me).
But anyway. These "reviews" and limited seat time impressions (let's be honest...20 minutes? They aren't reviews) truly mean nothing. Toyota rushed the journos through the entire process, you couldn't drive the car in day-to-day conditions, traffic, bumpy roads, etc. We still don't know enough about the car. All we know is that it handles well and it has a nice gearbox - that's it. It's the same bit of praise regurgitated over and over again. It tells me nothing, I still don't care. I want to see it driven in a real-world environment, not a glassy smooth track.
Edit: But I'll never say no to more power, as long as it is manageable - which is why I'll turn down Z06s, ZR1s, Mustang GT500s, and all of those other ludicrously overpowered American behemoths that either ride on archaic suspensions, or don't have nearly the sophistication and poise to keep you from killing yourself from lightly tapping on the throttle during a light drizzle over a minor bump. Give me the GT-R. Give me the Astons. Give me the R8 V10. Give me an LP560. Give me the LF-A.