I have two favorites.
First is Southern. Specifically the CNO&TP Railway out of Chattanooga. Many of my relatives worked for Southern, out of Chattanooga. Including my grandfather, who started his career as a fireman on steam and retired as an engineer in the diesel era.
My second is the Richmond, Fredericksburg. and Potomac. My first house was along the RF&P line. They took great pride in their railroad. Their engines and tracks were well maintained and every employee I met was proud of what they did. It was the little railroad that competed well with the big boys.