Probably the Samba Bus
As far as street cars go you might check the Standard Blitz D1 car, but I'd say the lowest car is the Pescarolo C60.. I think it "neutral" ride height is already like -10 and goes to -35 - -45
The older RUFs get -40 or something. I think the Mercedes 300SL can have -50.
The numbers on the ride height suspension setting have no correlation to the ground clearance of the car. It's just how far down from default it will allow you to lower it. Something with a suspension that can be adjusted down to -40 may not be as low as something that can be adjusted down to only -20 if the default height for the -20 car is much lower than the default height of the -40 car. For instance a Formula GT at default 0 ride height is already lower than many cars even when they are adjusted to as low as their suspension will go.
As for street cars, the '02 Honda NSX Type R gets way down there. Now, should it be done?