Class 802 in TPE livery for me. The Class 68 in TPE livery is great as well. A clean Class 395 or 373 is good as well.
It's really difficult to pick one loco but this has to be one of my favorites. E402b note this is the original livery, a red stripe between the headlights has been added later for visibilty but I like this one better. US Locos, I too love the streamlined US diesels, such as the F59PH I think they look really good. In the EU, I think the Vectron has to be the best, I really love its design, the Taurus being a close second.
The Pennsylvania T1 is a great looking locomotive, as were lots of other streamlined US steam locos. I'd also pick the original non-streamlined LNER P2s for their odd design, and perhaps the V2 and A1/3 as well. For me, the LMS Coronation (non-streamlined) is a close second. I never liked the A4.
Gonna give a second vote to the class 68, but I’d like to throw in the 158/159 as a contender, just a beautifully simple looking unit.
-Southern Pacific GS Class Locomotives, especially in the Daylight Paint -Brightline's Siemens Charger -UAC's Turbotrain -JR's 500 Series Shinkansen -JR's 700 Series Shinkansen EMD's F-Units
Another vote for the F-units from me. Going back to Britain, the Class 180 (especially in clean Grand Central livery) is a sight to behold, and I love the Kestrel concept. Too bad it was never preserved.