It wasnt my mod but my brother's: Possibly the LED/neon strips he put under the car. They never mounted properly, the rear strip never went on because the exhaust was in the way, and the front came off when a rabbit played chicken with the car and lost. One day he got fed up with them and just threw them into a bush on the side of the road! Was good fun at xmas time, as you could put them on a multi-colour setting
I cant call it my "worst" mod (as I like everything I've done so far), but the most painful (and expensive!) has been my Milltek. Spent too much money going from a non-res to a res, to finally having a custom centre-section put in place. Finally got it right now (I think!)