Since I don't consider a caboose or mantenance of way equipment (cranes, pile drivers, etc.) to be freight cars, it is really hard to choose. The more one researches the types the variations are amazing. Take the ubiquitous box car. Probably more variations on that than anything else. I like the ones with the holes in the end for loading long lumber. But all things considered I have to join the prior few posters who said Pickel Car. I am especially fond of the open sided ones. I did not realize how common they were until I was doing research for Space Mouse's layout design contest a few years ago. My entry then, of course, had a "Dillary" just for the pickel car.