Those iBud earphones. Not because they are the worst sonically but because they give people such a bad impression of what is considered good and bad about audio.
Also because they are ungodly uncomfortable with their relatively large earpiece and the way it is crammed into your ear. Also the build quality is terrible and in the past (a long time ago I might add) either the cable or the earpiece itself falls apart after a month or two. Also I see lots of people use them on the train or bus, which means that they must jack up the volume so they can hear the music over the noise, damaging their hearing over time. I mean seriously, sometimes I can hear their music while I have my TF10's in my ear and am listening to some quiet passages of a song. That is the worst part about those iBuds.
And yet time after time people keep going back to these earphones because they don't know about the better alternatives. There's going to be a lot more hearing impaired people in the future because of these iBuds, and that's why they are the worst headphones in the world.