I think the nicest thing a stranger has ever done for me was holding a door open or something similarly mundane. I don't have any other really nice or profound memories. Then again I have not gotten into any similarly bad accident as the OP (knock on wood)

I was in a car accident once, many years ago... nothing terrible but I was in shock (it was my first car accident) and in mild pain (bruises from the airbag and a headacke mostly). The other party came over and asked me if I had a phone. I was still in my car, trying to realize what happened. When I said that yes, I have a phone he said "then better call the police. the accident was your fault" (which for all I know was probably true), then he left to sit on the roadside and smoke a cigarette and scroll on his smartphone until the police and ambulance arrived, 15 minutes later. Because of him, they came with 4 or 5 extra vehicles, simply because I couldn't really answer their questions well ("how many people are in the other car? is anyone injuered besides you? are the cars still driveable or do they need to be towed?" all quesions I couldn't answer)

I overheard that he got a lecture from one of the cops later on, but still it was an experience that I don't want to make again anytime soon