My favorite car:
ex-New York Central sleeper lounge Chittanengo Falls.
She was originially built in 1939 by Pullman for the Commodore Vanderbilt. She was comparable in qualtiy to the cars on the 20th Century Limited. She is one of the very few remaining pre-war lightweights. (there are three pre-war pensy lightweights left...I can't remember if there are any other NYC cars). She has 3 compartments...each of which can be divided in half...a galley, and a beautiful lounge section. It currently sits on a stretch of abandon NYC branchline trackage east of Wapakoneta, OH. My father used to own her.
My second place goes to the Nickel Plate Business car #5 which sits in Lima, OH as part of the 779 display at Lincoln Park. She's a wooden car, 120+ years old, and she was originally built for Chance Le Plieu of the NYC.
My third choice is NKP caboose 141 which was recently restored by the FWRHS (765 crew). She was originally built as a 4 wheel car for the Lake Erie & Western sometime during the 1880s...no records survive to date her. She was lengthened around 1910. The interior of this caboose is very different than other cabooses. She doesn't have the usual set of cabinets under the cupola, but rather benches that run almost the length of the car...it's wide open inside the car.
I tend to think that most people would prefer non-revenue cars. It seems like most of the prize winning models are non-revenue cars...snow fighting equipment (rotaries, plows, spreaders, flangers), scale cars, dynamometer cars, cabooses, business cars, etc...