In the history of U2, you must name the worst haircut. Haircut may include facial hair, hats or head coverings, or anything else you can think of involving U2's hair.
The answer you are forbidden from is - Larry's mullet. I know not everyone would name that, but it just may be the most popular answer.
Feel free to provide accompanying pictures, caps, or icons. If you are on a dial-up service, consider that your warning.