for steam i'd say anything designed by Sir Nigel Gresley or SP's GS class.
For diesel i think any EMD/GM SD40-2 in FEC Champion, KCS Belle, DM&E, Conrail (quality logos), the new UPRR livery (lightning stripe w/baby wings) or SOO Line
for electric maybe GG1 or a Canadian National boxcab.
Despite being an example of the form follows function rule, with their Elesco feedwater heaters, flying (smokebox-mounted) air-pumps, deck-mounted headlights, and Vanderbilt tenders the Chesapeake & Ohio Railway's F-19 class heavy 4-6-2 locomotives were considered to be the manifestation of the Georgian steam locomotive and regarded by some as the prettiest locomotives the C&O ever owned.
In the mid-1940s, all five of the C&O class F-19 Pacifics were rebuilt as 4-6-4 class L-1 Hudsons at the railroad's shops in Huntington, WV. Four wore streamlining; one did not. Only one L-1 survived the torch -- #490, which today resides at the B&O Museum in Baltimore, MD.
The "big cylinder" is a Elesco feedwater heater, so it's a form follows function sort of thing. Adding a feedwater heater to a steam locomotive improved the thermodynamic efficiency of the locomotive, as the feedwater heater preheated the water (by using the heat from the exhausted steam flowing from the engines' cylinders) before it was fed into the boiler from the tender. The company name "Elesco" was taken from "Locomotive Superheater Company". "LSCo" is pronounced "El" "Es" "Co" or "Elesco".
On some steam locomotive, such as the NYC J1 Hudsons, the Elesco heaters were slightly sunken into the top of the locomotive's smokebox, and not nearly as prominent in appearance.