My favourite: the former West Australian Government Railways (WAGR), later known as Westrail. Sadly though, the freight arm of Westrail was sold to to a consortium of American company G&W and local Co-op Wesfarmers in 2000, becoming first 'Australian Western Railroad' and then 'Australian Railroad Group'.
Wesfarmers later sold out their share, leaving G&W to sell off the WA side of ARG to Queensland Railways, who retained the stupid 'ARG' name. There was rumours flying around that the WA government was in negotiations with QR to buy ARG back, but with the recent change of government here, it obviously won't eventuate.
Luckily though, the passenger side of Westrail remained in government hands, becoming the Public Transport Authority (PTA), organised into the Perth suburban rail network (Transperth Trains), and country rail (TransWA). But I will always prefer the old Westrail, with it's brightly painted locos and rolling stock (orange and blue, and yellow and blue respectively) and it's small but powerful fleet of locos.
Class I today though is either BNSF or UP. I grew up in CA, so they were quite a part of my railfanning as a youth. I liked SP and WP as a lil' kid, nice to see that UP has the Heritage locos, I hope they do more in the different paint schemes of the RRs they merged with, the all-black SP would be awesome in fresh paint! Oh yah and BNSF former rail colors too!
Newton & Northwestern (steam) which was previously the Boone, Rockwell City & Northwestern and later became the Ft. Dodge, Des Moines & Southern (electric and later diesel)