(みにくい) Literally ‘hard to look at’... physically ugly.
(しゅうあく) Hideous – more formal and more extreme than just みにくい. Also – unlike みにくい it’s more often used figuratively to mean ‘scandalous’: An ugly divorce, a scandalous rift between political factions.
(きしょくわるい) Ugly PLUS weird or uncanny: A slug. Also has the nuance of being contemptible BECAUSE ugly.
おぞましい Grisly or horrible crime or atrocity.