I chose the African challenge, it's by far the best of the 'cheap' challenges. The USA one was very good too.
I do agree though that you can't really categorise them as 'cheap car challenges', despite the fact they were kind of based on that premise. I think you'd have to choose from the ones where they have the scoreboard etc at the end, and have to do things for points, rather than drive from point A to point B.
Ignoring the America/Africa 'super challenges', the 10,000 pound supercars would get my vote.