The Steyr-Mannlicher M1905 is pretty ugly, but the ugliest duckling is definitely the Bergman No.3. Sadly its so ugly I can't find a picture of it online!
As for modern arms, I'd say the uglist one is whichever one the gang-banger thug is holding sideways while saying things like "s**t, dawg" and "what's up bros?" while sucking his teeth and trying to figure out which juvenile delinquint on the street is his offspring.