My favorite engine of all time is, what I consider to be the most beautiful steam engine built in America. Namely, Soutern Pacific's GS4 DAYLIGHT locomotive. 2nd is SP's Cabforward series, 3rd is N&W's Y6b and 4th UP's Challenger.
My favorite diesel loco is:- 1st Alco's PA-1 in SP Daylight colors, 2nd is the utiiliterian EMD SD40-2, 3rd is EMD's F59PH-2
ALCO and SD40-2 engines.
steam 4-8-8-4,2-8-8-2,2-6-6-6
diesel any cab units
electric GG1,R1
I'd also have to say the C424's the M636's and M420, HR412, HR616, and of course the CLC/FM Trainmaster, which CPR 8905 is the only preserved one left.