My collection of mostly scale Pennsylvania was started due to my love of steam. My favorite is the K-4. I have two K-4s and love to run them prototypically double headed pulling a long passenger train. I also run My Atlantic and G-5 with passenger trains. Occasionally, I take out the S-1 or T-1 for a great looking passenger consist.
For freight, I love the looks of my Consolidation pulling a string of Atlas billboard reefers. I also run M-1a, or J-1 if want to run "big stuff".
I also have a some electrics and diesels. My pride is, of course, my scale GG-1s. I have a MTH in brunswick green and a Williams in tuscan.