I have decided that the ugliest one-syllable word in the English language is “eighth.”
A gh right next to a th, and that awful Germanic A-sounding ei to boot. The whole word looks like something I half-barfed-half-coughed the morning after a night boozing.
Ugliest looking or ugliest sounding?
Actually I think eighth is a sweet soft-sounding word and interesting looking.
My vote is for ‘autochthonous,’ though ‘baluchitherium’ is also pretty ugly.
Humungous.
OK it’s not one syllable but even so it’s a horrible word
Dollop, easily.
Machicolation. I have no idea how to pronounce it, and it’s no longer in any dictionary.
I love eighth, because my sister missed it in a spelling bee once. AHAHAHAHA.
This is a very important point which should be clarified. OP seems to think that eighth has three sounds. OP also seems to think that there is something particularly “Germanic” about the vowel /ei/, though I don’t know why–it’s in all kinds of non-Germanic languages, such as Spanish, French, Italian, Korean, etc.
Ache
Isn’t spelled the way it sounds, has that harsh Germanic sound… and almost gives you an ache just to say it.
Ugliest 3 syllable: pluperfect
Plu sounds like a mix between poo and flu. It makes me feel ill just thinking about it. Ugh.
Poultice.
I’m glad it’s archaic and doesn’t come up in conversation much, because I’m creeped out by just the thought of saying it.
(Why no, I don’t much like the word “poultry”, either. Must be at least partly a sound thing.)
Puberty. As evidenced in Attack of the Killer Tomatoes.
Moist. It gives me shivers to say it. Similarly, Ointment.
Germanic A-sounding “ei”? In German, “ei” is pronounced like a long “i” in English, not like an “a” at all. 
I like the word “eighth”. Offhand, I can’t think of the ugliest word in the English language (for me).
Halitosis?
Meal.
Runner up:
Vagina
Except for dictionary.com, wikipedia, tfd.com, Merriam-Webster?
Pulchritude. Such an ugly word, and its actual meaning is the opposite.
My vote is for “rural.” It’s impossible to say!
Belly, bean, and happy, to start with.
I like it. It makes me think of fresh brownies.