I'm thinking the Benz Patent Motor Wagen, but that isn't in GTPSP.
I'm also thinking the Ferrari F40, but that also isn't in GTPSP.
The next thing on my list is the Countach, but you already said that.
Maybe the 300SL? Is that in GTPSP?
Other than that, perhaps the Mercedes-Benz SLR McLaren '03?