Well thats an easy an obvious question..... The grasshopper of course.
All kidding aside, the car/truck i run which does the best on grass is probably the Associated 8.2 e, followed by the savage flux of course.
I think any offroad 4 wheel drive 1/8 will do great on grass followed by the monster trucks which are 4x4 and then the 2 wheel drive off roaders. The trick here is ground clearance and good tires.
Any 1/8 scale buggy will rip through grass with no problem imo. My Associated rc8.2e would abuse my Savage flux if they where to race on a made up grass track as a local park no problem.