Get a loving R8. Now.
If you tune it right, you can beat most cars online with a little skill, minus Formula and Racing. Also a well tuned R8 blows away the Super Car event with ease.
Pic of my R8 taken ingame:
Edit: That's also the cheap R8, tuned to about 750HP. And I can pretty much beat everything with it + softest tires. Mine tops out at around 340km/h. And rockets out of corners.