Well, asking for a "best" car choice will get a lot of different (and usually very passionate) responses. It sounds like you might want to spend some time with the tweaks and upgrades? Like, take down the Viper from the 1000+ HP you have it at. Instead, try putting in a full racing transmission and adjust the gears for the tracks.
It's not that hard, especially in GT3. Here's an easy way to do it on any car. Go to the trans. setting, go down to the auto bar and move it down or up. Then take it out on a track, say, for a qualifying lap if you're in a race.
Now, the car should redline in top gear shortly before you begin braking after the longest straight. There are some exceptions depending on the track, but that's a good general rule.
If the ratios are too long, for example, and you're just hitting top gear when you start braking, go back to settings and shorten them (move the slider to the left) and try again. Pretty soon, it'll become fairly easy to know about where to adjust your settings.
This way, you don't really need 1000 HP to win at the Test Course, just set really wide ratios. (You should lower downforce, if possible, but just work on the ratios first). In Monaco/Cote D'Azur on the other hand, I set it to redline about the time I come out of the open tunnel.
I hope this helps.
And if you really want to know my personal fav car in the game, it's the NSX. But that's just me. You'll find a fav, too.