In no particular order:
The Japanese have always had trouble inventing their own designs, usually they copy the designs from the west. On the rare occasions when they go off on their own, tragedy results. Such is the case with the Nambu pistols. Designed by Kijiro Nambu just after the turn of the century. Rumor has it the Mauser C96 Broomhandle had some influence on the design. Looking at the original Nambu, you can see the resemblance to the Luger, one of the most beautiful guns ever made....
I guess it was too pretty. The mucked it up as the designs progressed. The type 14 (4th variation) was adopted by the Japanese Army in 1927
The ungainly Mauser C96 Broomhandle also makes the list, I know a lot of people like this gun and it has great historical significance, but it isn't winning any beauty contests!
Another gun with great historical significance, and the inspiration for the P08 Luger, the Borchardt C93 was ugly with a capital U.
World War II was a very costly war, all parties involved found themselves strapped for money to build more small arms. The United States Army's answer to the problem resulted in the M3 Grease Gun, it was given the name because it resembled the same grease gun the Army mechanics used to lubricate machinery.
Moving to the more modern guns, the HK VP-70 was one of the first polymer framed guns, it looks like it would be comfortable to shoot, but it lacks visual appeal. It almost looks like a piece of "minimalist" art.
The VZ 61 Scorpion makes the list, it appears as if the designers were only interested it making it work....
The Chiappa Rhino looks about as lovely as its namesake. it's too bad too, because the Meteba revolver that this design is partially based on was a pretty good looking gun.
Hi-Point Firearms makes guns that work, and are accurate as well, but they are about the ugliest things ever made.
Rounding out the list is this unlikely gun.
The Winchester model 94 rifle is one of the sexiest guns made, no argument there. Mossberg went and made a great looking copy of the Winchester.......Then they went and committed this act of blasphemy. ATI (the stock maker) should have slapped Mossberg back to their senses when they were approached with this idea.