Are steam locomotives allowed? If so there are plenty of contenders in that category. I'll start with a couple e.g. Bulleid Q1, BR Crosti Boilered 9F....etc.
No way! The Q1s were not ugly at all, a bit weird, but not ugly. I agree, however, about the Crosti 9Fs; they were appalling. The Great Western streamlined Castle was a bad joke, but I heard somewhere that this was the intention: showing that there was no need to embellish a very attractive design by adding streamlining.
A bit more controversial would be my choice of streamlined Stanier Pacifics. Far too bulbous to be attractive; someone made the apt comment when they first appeared that they looked like an upturned bath-tub. They looked superb after the cladding was removed. I also find the unrebuilt Patriots and Royal Scots quite ugly with their huge untapered boiler and silly little chimney. They were much better when rebuilt with a tapered boiler. I would have preferred the new-built Patriot to have been done as a rebuilt one.
As for modern stock, I've just looked at pictures of the 380 and 385 -- unbelievably awful; I wish I hadn't looked at them. I'm just glad that they are kept well north of the border.
The ends of the 455s on the Southern look like the company couldn't be bothered to finish off the job of rebuilding them. The original SWT 458s were pretty ugly with the 'wardrobe doors' at the ends; the rebuilt end is a lot better, but I agree with what others have written about the mismatched window surrounds on the carriage sides.
