Otherwise, in your terms, what car have you seen on Forza affected you so much? Or possibly was it a car of yours that was so awesome it made an impact on you? As for my choices..
CobraTuner's Andy Sapp BMW E46 (FM2)
(Not actually CobraTuner's, since he didn't paint this car on FM4)
The Reasons: I remember when he first made this car, and how amped everybody was to get it. It was an LE, and EVERYBODY wanted it. I remember I had one the very first car he sold, on the basis of who I was bidding against's Xbox froze. I got it for a deal at 80 Mill (which is pretty bad, the economy was awful in FM2). It was basically a fancy paperweight though, since every time I took it out in a public room made the game freeze.
Boppinz Maruta-Goya S15 (FM2)
The Reasons: The moment I saw the car, I thought it was heinous. It started to grow on me though, it was the first car, other than N.O.B.'s S15 to hook me onto D1 cars. It just looked...different. I liked the multicolor flames on it, and just the general flow of the car. Not to mention Boppinz destroyed it. He did such a great job, and the prices that the car commended didn't do it justice. Multi-billion prices I want to say.
That's all I can think of off the top of my head. Yours?