Forza Horizon

There are 30 street races divided between 3 locations to win for this achievement. The locations and races unlock as you progress in the game. I did not unlock the final location until I had completed several Horizon events after getting the purple wristband, and didn't unlock the final race for that location until I had completed most of the gold level events.

The street races are point to point races with traffic and no race barriers, but are still not all that challenging. I completed them all with no assists and had no real trouble.

The locations and race levels are:

Carson:
C
B
A
B
A
A
B
B
A
A

Red Rock:
A
B
A
S
A
A
S
S
R3
S

Gladstone:
S
A
S
A
S
R3
S
R2
R3
R2

As far as cars go, any one in the correct class can race but the ones I personally found the best were as follows:

C: 1970 Ford Mustange Boss 429 @ C400
I'm sure there are better choices for this class, but seeing as there is only one of them I didn't bother looking into it.

B: 1983 Audi Sport Quattro @ B500
AWD. You can rev this to the limit at the start and not smoke the tires, giving you a good boost at the start.

A: 2002 Nissan Skyling GT-R V-Spec II @ A600
This is one of my favorite cars because it is so easy to drive. You can absolutely thrash on it at this level and it will fight you to lose traction the entire way. Also a decent choice for the B class races. Another car that you can rev to the limit at the start and not smoke the tires off the line.

S: 2012 Nissan GT-R Black Edition @ S700
Easily obtained from the Marko Star Showdown, AWD, lots of power, what's not to love?

R3: 2010 Dodge Viper SRT10 ACR-X @ R3-800

R2: 2012 Lamborghini Aventador LP700-4 @ R2-897
Another great AWD car, and one of the loyalty cars. It is also a very good choice for the R3 races.

If you feel the need to down vote, please be courteous enough to leave a comment telling me what I did wrong.

Street races are located on the map with a square box with the dollar sign in the middle. You will be notified were they are, aswell as being updated when new races are available, as you progress through the wristbands.

The best cars i felt were the best:-

C: Ford Mustange Boss 429
B: Audi Sport Quattro .
A: Nissan Skyling GT-R V-Spec II .
S: Ferrari 430 Scuderia
R3:Lamborghini LP-400
R2: Lamborghini Diablo

Lambos are by the best at the R3 and R2 levels due to the AWD capability.
Any questions please feel free to contact me!