I think my favorite right now is the PRR X29. Atlas has done a great model of this car in many modifications as mentioned above.
The PRR had about 40,000 of this class car and it was the prototype design for the 1923 ARA car which many other roads had. 40,000 cars is more than many RRs had on their roster.
This is a small car - it is 40ft long, but I think 9 or 9 1/2 feet high inside vs 10 or 10 1/2 for more modern cars. It is an ideal car for modeling the transition era and earlier because there were so many. Much manufacturing in the North East USA was along Pennsy tracks, so many products were shipped in X29 boxcars to destinations all over the country. I have seen photos of them on sidings in California for example. They could show up on a model of any RR or area of the country.
And as a PRR fan, the most important thing is that this is a signature car that just says "Pennsylvania RR"
