There is a lot to consider here with this question. Open wheel? Closed wheel? What chassis? What era?
The lowest best handling body would be the current MegaG F1 car, but you are restricted to a LWB MegaG chassis.
My favorite overall magnet racing closed wheel sport car is the Tomy Porsche 959.
It is not the lightest body, but has a low center of gravity, is well balanced, and can take
a beating in a race. I mean it tends to stay in its lane through others crashes and marshalling miscues.
The lightest closed wheel cars are the AFX Datsun 240z, 71 Camaro, and Ferrari Daytona.
These tend to be favorites among racers because of there small mass.
The AFX Porsche 917 body is also a good one, and is available cheap.