All of them, Katie! PRR, B&O, P&LE, Wild Mary, Shawmut, Union, UP, Santa Fe, Espee, D&RGW, Burlington, they are all favorites.
However, what I model (loosely) is the contemporary Oil Creek & Titusville. That is to say, the back-story of the Plywood Empire Route is patterned after the OC&T. Titusvile was situated on a Class I Mainline (PRR, Pittsburgh-Buffalo) until the entire route was abandoned. In order to maintain rail service to their enterprises local businessmen got together and created a railroad by acquiring several miles of that abandoned railroad in order to connect to the WNY&P at Rynd Farm. The OC&T moves 1,000 cars a year (about three a day) making it an ideal prototype for a shortline model railroad. Even better, every train is a mixed train (passengers and freight). You see, the OC&T subsidizes it's freight revenue with ticket sales to tourists.
I have found that a mixed train doesn't work for me because O gauge passenger equipment is under scale size (as well as being short Lionel and K-Line aluminum cars are slightly narrow and slightly low) by just enough to clash visually. Thus, tourist service on the Plywood Empire route is provided with a Dinner Train a la the Black Hills Central Railroad.
I say loosely patterned after the OC&T because most freight equipment on the Plywood Empire Route is Fallen Flags, which doesn't fit in a contemporary operation but hey, it's my Pike and it works for me. 
