All time ugly were the 6 and 104 of the Pacific coast. These two 4-4-0's had pencil sized boilers jacked up more than a boiler width above the cylinders. They looked like they were ready to fall over.
D&RG 432 (later to be the very attractive 375) with the huge three window cab has to be another choice. As difficult to run as she was ugly - had to get out of your seat to go up and open or close the throttle.
Also, to turn the tables from the silk purse I would have to nominate the sow's ear - RGS 20 before she got the 25's tender. U G L Y !