New Haven, Pennsylvania, NY Central, Jersey Central, Baltimore and Ohio, and Erie, for various reasons.
New Haven because I took their trains growing up in Mt. Vernon, NY to NYC when my mother (who did not drive) took me with her when she went shopping at Macy's, Lord & Taylor's, Saks 5th Avenue, and the other fine stores in Manhattan. She loved Lionel trains so we would see them in the toy department at Macy's. Also, I always loved the look of the New Haven McGuiness livery and the silver aluminum passenger cars.
The Pennsylvania for a several reasons. As a child, my mother and I would go to Penn Station to visit my dear aunt and uncle who lived in Pennington, NJ. We went to Penn Station and took the train to go there. Penn Station was enormous and awesome. I knew nothing about architecture as a child, but I could sense the greatness and beauty of the architecture there, and the beauty of the light streaming through the huge ceiling windows. The GG1s there were equally awesome, reeking with power. I also loved getting a soda and cheese and peanut butter crackers riding on the train from Penn Station to Trenton or Princeton Junction. No such snacks were served back then on the commuter trains from Mt. Vernon to NYC.
Another reason I loved the PRR are the endless freight trains I witnessed when they rumbled past the swimming pool area at Hopewell Country Club, not far from where my aunt and uncle lived. We would be sitting by the pool, count the coal and oil tanker cars, and I wondered how the locomotive(s) could pull such a long train.
The NY Central because that's the train I took after my family and I moved to the Fleetwood section of Mt. Vernon, NY, including when I took the train to go to college. The NY Central diesels were very impressive.
The Jersey Central because I love the look of the Blue Comet model trains. My favorite steamer is the LC+ Jersey Central Pacific with its lovely light blue livery.
The Baltimore and Ohio because as a child I always wanted a Lionel diesel engine, and finally got one when, at the age of 10 or 11, my father bought me a Lionel B&O 44 tonner, which I still have. Also, I like the look of the B&O livery (blue is my favorite color), and that the B&O was the first railroad (or one of the first railroads) in the USA. I even like the sound of the name "Baltimore."
Come to think of it, I also really like the sound of the names "Pennsylvania" and "New Haven."
The Erie Railroad also because of its name, "Erie," which is such a cool name, IMO.
These railroads have a lot of aesthetic appeal to me. Arnold