My take on the worst cars is a bit different.
3 come to mind.
1) Spoon S2000 Race Car.
It's such a great car that I am super unhappy with its major lack of power. This thing can't even keep up with the normal road versions, none the less, the tuners like the mugen or the spoon.
2) Jaguar XJ220 Race Car and Road Car.
This tag team is just about the biggest disappointment in the game. In real life, the XJ220 road is awesome... Flat out awesome. I don't even know if there is a real life race version, but in keeping with tradition, the Race version of this car should be on pace with some of the fastest in the game.
The race version is so off pace it is not even funny. But with the engine, the drivetrain, the dimensions, etc. this car should have been a top runner. Instead, we got a big hunk of metal that drives like crap. A major let down indeed.
3) Lister V12 Storm
This may very well be the let down that keeps the jag from taking top honors as the biggest let down in the game.
The storm consistantly did well in the British GT championship as well as the FIA GT championship. It has an awesome engine, and all of the attributes that would create an awesome race car in any other form.
However, the in game lister is slow, heavy and handles like crap. This is indeed a horrible thing.
I would have to say the thing that makes many of these cars the worst cars in the game is the fact that common sense based on real life knowledge and theoretical ideas about "what should be" make these cars total disappointments and far less valuable racers than they should be.