For the 19th century I would have to say the Iconic 4-4-0 American. It had variations and worked from mainlines to short runs. The steam engine dearest to me though is the ALCO 3000 series better known as the Challengers. Big, tough, just awesome to look at but then hey, ALCO....nuff said.
Ah, good choice, real workhorses.
My favorite steam engine would have to be the Baldwin K-27 2-8-2. Also known as the "Mud Hen". They were originally built for the Denver and Rio Grande Western railroad. There are 3 K-27's still in operation today, one of them is at the Huckleberry Railroad in Flint, Michigan, which is my hometown.
"73" from https://wpyr.com/