Even though these are not my photos, and I'm surprised it was only mentioned once near the beginning of this topic, but SP 4449 or any Daylight color GS series to me the most beautiful steam locomotive made. As to the 4449, I prefer the large Southern Pacific on the tender then the small Southern Pacific Lines. As to a 2nd, 3rd & 4th favorite for me, is the NYC Hudson, Empire State Express Streamline Hudson & Niagara.
And for added bonus for a diesel, these 3 in order are my favorite below for beauty in my opinion for color, not counting engine style or body. All 3 photos below, I took myself.
Plus this below, which I took this photo, to add for a final bonus.