A great one-liner insult is about delivery, humor, and debasement. Lines like "you must have been born on a highway, because that's where most accidents happen" and "if I wanted to kill myself, I'd climb your ego and jump to your IQ" are great examples of witty one-liner insults.
The classic "yo mama" joke is a great way to insult a person with a one-liner. Dating back to ancient Babylon, mothers have been the inspiration for insults for over 3,500 years.
A great "yo mama" one-liner insult uses imagery, allusion, puns, similes, and other joke devices to get the point across. For example, "yo mama is ugly" is not as effective as "yo mama is so ugly that when she joined an ugly contest, they said "sorry, no professionals." Try to be creative when insulting your enemy's mother's appearance. Why is she ugly? "Yo mama's so hairy, she shaves with a weed-wacker."
Appearance isn't the only attribute you can insult, either. Try, "yo mama's so stupid, she thought Thailand was a men's clothing store," or "yo mama's so old, her candles cost more than her birthday cake." Of course, any good "yo mama" joke can be altered to insult a brother, sister, father, or even the person you are directing it to, opening up great possibilities for one-liner insults.