While this may be highly personalized:
British: Mallard. And Cantlie's KZ 4-8-4.
American: J1 Hudson, PRR K4, Niagara. Recently PRR T1, of course. I am a 4-8-4 proponent in general, and French 242A1 in particular. German 05 (and, honorarily although misdesigned, 06). Japanese C62.
NZR: the Ks. Funny headlight placement and all. (I am also a Garratt fan, dangerous though that might be, and a great admirer of the As).
The 2-4-2s are notable for another reason. Dug out of streams and completely rebuilt. More reasons why I love NZ.
Oz C36. (Honorable mention to the latter Garratts (NOT the ASG), the T1 lookalike, and the S-class.)