Ugly, hideous, ill-favored, unsightly are comparable when they mean contrary to what is beautiful or pleasing especially to the sight.
Ugly is the comprehensive term which may apply not only to what is distasteful to the sight but also to the hearing or occasionally to another sense or to whatever gives rise to repulsion, repugnance, dread, or extreme moral distaste in the mind.
Hideous carries an even stronger implication of the personal impression produced by something ugly by stressing the suggestions of induced horror or loathing; it may on occasion be applied to something which, without regard to any surface ugliness, arouses intense personal distaste.
Ill-favored applies especially to the features or to the general aspect or appearance of a person or sometimes an animal that may be described as ugly; the word emphasizes the unpleasing or disagreeable character, so far as the eyes are concerned, but seldom suggests marked distaste, repugnance, or dread.
Unsightly usually refers to things, especially material things, upon which the eye dwells with no pleasure; it often is used in place of ugly when a connotation of distaste is not strongly marked.